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.
Filenames with characters that need multiple bytes in UTF-8, and/or
control characters like combining diacritics, would break the grid.
`StrExt::width` seems to do the right thing, and there's conveniently a
function here (which was otherwise unused) to call that.
- 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.
