emacs: Add more functions to clean up text/plain parts

Add:
- notmuch-wash-wrap-long-lines: Wrap lines longer than the width of
  the current window whilst maintaining any citation prefix.
- notmuch-wash-tidy-citations: Tidy up citations by:
  - compress repeated otherwise blank citation lines,
  - remove otherwise blank citation lines at the head and tail of a
    citation,
- notmuch-wash-elide-blank-lines: Compress repeated blank lines and
  remove leading and trailing blank lines.

None of these is enabled by default - add them to
`notmuch-show-insert-text/plain-hook' to use.

Reviewed-by: Carl Worth <cworth@cworth.org>: I previously committed a
stale version of this patch.