This commit adds --grid, which, when used with --long, will split the details into multiple columns. Currently this is just 2 columns, but in the future it will be based on the width of the terminal.

In order to do this, I had to do two things:

1. Add a `links` parameter to the filename function, which disables the printing of the arrow and link target in the details view. When this is active, the columns get way too large, and it becomes not worth it.
2. Change the `print_table` function from actually printing the table to stdout to returning a list of `Cells` based on the table. This list then gets its width measured to calculate the width of the resulting table.

Finally! The benefit of having all the field-rendering code (in details.rs) separate from the value-getting code (in file.rs) is that rendering them can be tested again.

It's now in the locals of the Table struct, and didn't really belong in the column anyway.

The benefit of having these all as separate methods instead of using the same trait is that we can just pass parameters in like this!

And move all the rendering, ansi_term, colourful stuff into the output modules, which is the only place they should be used!

Colours are now disabled when output is not to a terminal. Fixes #53!

This required some internal restructuring - colours are now in their own object that gets passed around everywhere it's needed.

Exa now uses the new IO, Path, and Filesystem libraries that have been out for a while now.

Unfortunately, the new libraries don't *entirely* cover the range of the old libraries just yet: in particular, to become more cross-platform, the data in `UnstableFileStat` isn't available in the Unix `MetadataExt` yet. Much of this is contained in rust-lang/rfcs#1044 (which is due to be implemented in rust-lang/rust#14711), but it's not *entirely* there yet.

As such, this commits a serious loss of functionality: no symlink viewing, no hard links or blocks, or users or groups. Also, some of the code could now be optimised. I just wanted to commit this to sort out most of the 'teething problems' of having a different path system in advance.

Here's an example problem that took ages to fix for you, just because you read this far: when I first got exa to compile, it worked mostly fine, except calling `exa` by itself didn't list the current directory. I traced where the command-line options were being generated, to where files and directories were sorted, to where the threads were spawned... and the problem turned out to be that it was using the full path as the file name, rather than just the last component, and these paths happened to begin with `.`, so it thought they were dotfiles.

Fixes #55.

Still missing a few Beta features, but it compiles!

- Copy requires Clone
- current_dir returns a Path now
- num_cpus moved to a crate

This module provides feature-specific implementations, and also dummy implementations for when they aren't supported by the system or OS.

Doing it this way limits all the #[cfg(feature)] annotations, as we can now just include the module or not.

Closes #47

Closes #27.

So move it as part of 'recurse's option.

Move most of the heavy lifting into a Table struct, which doesn't govern how the resulting table is *created*, only how it's *displayed*.

Everybody does it, right?

See #24

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

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.