Introduce X-STOP

Require the new keyword 'X-STOP' to mark where to stop looking for keywords in messages.

This changes cares only for the syntax, not the semantics (e.g. it doesn't produce errors if x-stop is not present). The semantic checks will be implemented in the new keyword handlers runner (see !227 (merged)).

Parsing the arguments follows the same rules as before: the lines are downcased and split into words. Plugins/Keyword-handlers that need access to unmodified data must read that data from the remaining body or attachments (as they do already).

Closes #373 (closed)