Support hooking fish shell.

This is complicated by:

• No support for something like ${(z)var} to automatically handle quoted parts.
• Redirections and pipes involving blocks (like for-loops) buffer output fish-shell/fish-shell#1396
• No support for a "pre-exec" bind/event, only bind \r (bind enter) which gets triggered for things like pressing enter on tab-complete, or when an incomplete command line is entered. (However we can determine if command line is valid)

@l0kod I would like to work on this