This change does a few things:

- Fixes composing
- Fixes extracting attachments
- Adds the ability to send email with the mail rubygem.
Fix sending email via rubygem mail

Had to intern all the strings in the hash for it to work properly
Exclude mails tagged with any tag specified in
notmuch config get search.exclude_tags
in folders list and search list.
Prevent search_show_thread() render all messages in
the notmuch db, if invoked from an empty search list,
e.g. search for a non-existent tag and press <Enter>
on the empty line.