Hi,

this adds a new custom variable, `notmuch-mua-subject-regexp`, and an
associated function, `notmuch-mua-subject-check`, to warn the user when
the subject contains potentially troublesome things (e.g., nothing at
all). The idea is the same as `notmuch-mua-attachment-regexp`—which has
saved my skin quite a few times in the past—but for the subject instead
of an attachment. By default, it checks for empty subjects, as that
seems to be a reasonable thing to safeguard against.

I've not found any NEWS entry (nor any documentation) for
`notmuch-mua-attachment-regexp`, so I wasn't sure whether to add any
myself. So far, I haven't done so, but that can of course quickly be
fixed.

Best,
  Tony