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.

This was intended from the start, but the dead_code checker was dulled by the module being public!

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.

Still missing a few Beta features, but it compiles!

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

- Rename std::io to std::old_io

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

- Prefer iter over into_iter where appropriate
- Cut down on cloning

I can't complain, because the breaking changes were caused by my pull request...

Yeah, I broke my own code again.

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

I truly didn't realise this wasn't already done until someone sent me a .BMP file.

There doesn't seem to be an io::FileType entry for character special
devices or sockets, so these fall under the io::UnknownType entry. Which
also gets highlighted in yellow, for precisely that reason.

I'd much rather have this separate, as it has the definite potential to
balloon up to a huge size and end up making the other parts of the file
module hard to read. But on the other hand, it meant making a few more
methods public on file... readability over cleanliness, I guess.