Improve zsh completion for field names

[lucc]

There are two places where it makes sense to complete field names on the command line:

• the -F option
• the search query

This is currently not implemented as it needs some zsh completion magic.
The correct field names to complete can be found in khard.cli.field_argument and khard.query.parse respectively.

(This is a follow up of #131 and #246.)