Basic searching is supported with the following:
- read messages
- unread messages
- from addresses
- text in body
- text in subject
- text in all

The implementation loops through all messages in the selected directory.
It tries to be smart by detecting which parts of each message the search
query needs to use and only loads these from the filesystem.

Syncs back special notmuch tag like unread to the underlying maildir store

For some reason the current code frequently segfaults due to an
invalid C memory address. This commit mediates that by never keeping an object
alive longer than absolutely necessary.

This adds the event type as well as the command implementation, but no backend
supports it yet.

This patch adds search behaviour to allow searching in the body of the
messages, the entire text (body + header), and searching just the from
header.

Signed-off-by: Wagner Riffel <wgrriffel@gmail.com>