Add a man page entry for exa

Fix a missing '/' in symlink targets

In cases where symlink targets were more than a single directory down,
exa did not print the '/' targets when separating directories, resulting
in the following output:

    symlink => dirAdirBdirC/file

Instead of

    symlink => dirA/dirB/dirC/file

By adding a '/' character after each component of the filename, this
error is fixed.

This patch adds a manual page that can be parsed and outputted by the
`man` command. The help command is useful, but many UNIX users have a
certain reflex to check for manual pages. It would be awesome to be able
to type `man exa` and see some output! Some notes:

 * This manual page was hand crafted. I had hoped to find some Rust
   utility that could generate manual pages to prevent having to change
   this file whenever changes to options are made. There are tools to
   write manual pages in a nicer syntax and then convert them into the
   groff syntax, but I hesitated to add more tools to the build process.
   This might make the file more difficult to maintain, but unless exa's
   signature changes frequently, it should be okay.
 * The manual page isn't automatically copied into the
   /usr/local/share/man/man1 directory, where it would need to be for
   `man` to read it. My first inkling to have installation be a nice
   process would be to craft a Makefile that will run `cargo build
   --release`, copy the binary into /usr/local/bin, and copy the man
   page into the aforementioned directory. I can handle that in this PR
   or another one if you like the idea of a Makefile to ease
   installation!
Signed-off-by: NDavid Celis <me@davidcel.is>

See #24

Conflicts:
	src/file.rs

Fixes #26.

The old implementation blindly assumed that a symlink target would have a directory compoment, which the current directory, parent directory, and root directory technically don't have.

Fixes #20.

Optimize Builds

lto makes exa slightly faster and much smaller. I specify the other
options to make them explicit.

Note: lto makes compiling take quite a bit longer.

Improve matching of Git status entries to files

The logic of the previous version wasn't correct. Also, presuming
natural ordering of full filenames is still reasonable when the
extensions are identical.

Get rid of explicit `as_slice()` calls

Fix the column alignment

The challenge is that the paths returned from libgit2's status listing
are from the perspective of the Git repository and thus effectively
relative to the working tree root, while the other paths we're
manipulating are (potentially) relative to our current working
directory. So, if those two aren't identical (if running from outside
the working tree, or from a subdirectory), the paths won't match up.

A reasonably reliable way around this is to resolve both types of paths
to absolute paths before comparing them. This fixes #15 at a basic
level, anyway.

What still doesn't work: referring to the working tree or one of its
descendants via a symlink. For that, we'd probably need to fully resolve
symlinks in the file path.

(The unwrap_or()'s are messy and will probably just result in missing
status information, but then, what information could you hope to get
without having both a current working directory and a Git working tree?)

Use file_name_width in grid

Filenames with characters that need multiple bytes in UTF-8, and/or
control characters like combining diacritics, would break the grid.
`StrExt::width` seems to do the right thing, and there's conveniently a
function here (which was otherwise unused) to call that.

These were completely missing because I forgot about them... I added sorting functionality before I added date functionality, but it turns out I didn't even need the datetime library to do this!

However, this implementation feels incomplete. If you sort by the accessed date, it still displays the modified date, so the dates still look out of order. If I were to change the date column for the user, though, then there would still be no header to signify that the column has changed. I'm not sure how to proceed with this, but this is still missing functionality, so in it goes.

FIX bitflags depedency

Fixes #7.

- Feature flags changes (old_io, old_path)
- Slice syntax changes ([] -> [..])