Playing with the awesome Prompt Toolkit

Let's see what we can do based on the tutorial and the shell.

Amazingly easy to set up a command line with (some) colouring and some completion

• Consider using compgen to generate the completion words instead of traversing $PATH myself (although that was fun)

• Wire up a shell to process the input

• word_completer apparently treats '-' as a word separator so doesn't properly complete commands-like-this.