- env is no longer feature gated
- exit_status is!

current_dir returns a new PathBuf, but getcwd returns an old Path. One day I'll have to upgrade to the new Path library. One day.

Fixes #41.

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

- Update package versions
- locale::default -> locale::english
- Argument handling changes

This has been mostly done with changes in the datetime crate's suddenly
supporting locales.

It's still important that the user's locale is touched only once and
cached from that point on, so a struct in output::details has been made
public, along with that module. This will change later as that object
gains more and more uses thoughout the codes.

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.

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.

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.

- Rename std::io to std::old_io

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

- uint -> usize
- getopts Cargo library
- replace feature gates with unstable APIs

Also, remove dependency on the Regex library by replacing the one place it was used with standard code that should hopefully be faster anyway.