RFC: add space as argument separator

[David Bremner]

I guess the  main question here is whether we  should support space as
seperators for boolean arguments as well.  I'm not really sure if it's
possible to dissambiguate e.g.

% notmuch show --decrypt false friends

It's not clear if false is a search term or an argument to decrypt.

Personally I can live with requiring = or : for cases where a boolean
option needs to be set.

The other issue is completion. I guess this probably breaks bash / zsh
completion (or at least using ' ' does); I have not tested.