Sanitize "Subject:" and "Author:" fields to not contain control characters in notmuch...

When a Subject field contained encoded CRLF sequences, these sequences
would appear unfiltered in the output of notmuch search. This confused
the notmuch emacs interface leading to "Unexpected Output"
messages. This is now fixed by replacing all characters with ASCII
code less than 32 with a question mark.

Signed-off-by: Jameson Graef Rollins <jrollins@finestructure.net>