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.

Fixes #7.

Use the `locale` crate as a dependency to read in the set
thousands-separator character, and pass this to the file size column,
which uses it to add the separators in.

en_GB uses ","
fr_FR uses "" and just displays the numbers in one go.

Otherwise, just display the hour and minute.

Add options -u, -U, and -m as shorthand options for displaying different
types of timestamp, not just one. It's possible to have more than one by
specifying more than one of these shorthands, but *not* when used with
--time, as this is only meant to display one at a time.

Using the datetime crate, add an extra column to the --long view that
prints out the modified, accessed, or created timestamp for each file.
Also, let the user pick which one they want to see based on the --time
command-line option.

- std::os -> std::env

- Turn the views and main program loop into structs, rather than just as one gigantic function
- Separate views into their own files

The addition of the git column and the tree view meant that a lot of functions now just took extra arguments that didn't seem to fit. For example, it didn't really work to have only one 'view' method that printed out everything, as the different view options now all take different parameters.

Using `retain` instead of `filter` means we can just re-use the same Vec.

FileName was always a special-cased column, as it was assumed to be the last column in the output. Now, it's explicitly marked as such. This allows the hash marks to be placed before the filename, rather than at the start of the line.

There's still a lot to do, but this is actually *something*. The tree hierarchy is displayed using hashes at the start of a line. I want to have it just before the filename, but this will need some changes to the way that columns are handled.

You can now have different columns per directory. So now, the Git column only appears when there's a Git repository in the current directory.

This is something that I've long wanted to add. It uses libgit2 as an optional dependency.