notmuch new: Don't index uuencoded data.

With modern MIME attachments, we're already avoiding indexing the
attachments. But for old-school uuencoded data in the mail, we have
been directly indexing the encoded data as terms, (which is not useful
at all---nobody will ever ytry to search based on the seemingly random
uuencoded data).

Additionally, indexing a modestly large uuencoded file seems to make
Xapian go insane, (consuming *lots* of memory).

We fix both problems by detecting uuencoded content and not performing
any indexing of it.