- 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.

Extensions aren't ever displayed in lowercase, just compared case-insensitively, so this makes sense.

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

std::str changes, and the way macros are expanded.

I'm serious, the more functionality I can use external crates for, the better.

Also, re-use the stat result from directory-checking.

This finally fixes the issue where trying to list a file causes a crash. Also, tidy up some of the uses of references.