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

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 and remove blank lines between attribution statements and
    the citation,
- notmuch-wash-compress-blanks: Compress repeated blank lines and
  remove leading and trailing blank lines.

Enable `notmuch-wash-tidy-citations' and
`notmuch-wash-compress-blanks' by default by adding them to
`notmuch-show-insert-text/plain-hook'. `notmuch-wash-wrap-long-lines'
is not enabled by default.

If `notmuch-wash-wrap-long-lines' is enabled, word wrapping of the
buffer leads to an unappealing display of text, so provide a function
to disable it and add it to the list of `notmuch-show-mode' hook
functions.