256-colour support

Add a Fixed(u8) constructor to Colour, which represents the 256 colours
that some terminals support. This means we can:

- stop using black bold to mean grey, which looks weird on terminals
  that haven't been set up to use it;
- support a *lot* more file type colours.

I'm a little suspicious of how much string allocation is being done in
colours.rs, but that's a problem for another time.