Resolve "matching keywords in body is too strict"

This changes the way how we parse for keywords to the following approach:

1. Ignore any empty/pure-whitespace lines at the beginning of the first plain_text_part message.
2. If you match something /^X-.*/ start matching for plugins, else abort.
3. Once you matched a plugin, but you have a new non-empty/pure-whitespace line starting with something different than /^X-.*/ abort looking for plugin names.

This fixes the most common problems with lines starting with X- further down in the body of an email, while keeping all of the existing behavior and functionality.

Any further improvements to the code base and the parsing of mails as discussed in #249 (closed) is postponed to future work. Tis work should go together with work in #359 (closed) which will allow to change the structure of plugins.

Partially implements #249 (closed)