[Evolution] External filtering in Composer

[Patrick O'Callaghan <poc usb ve>]

I don't think this is currently available, but it
would be nice:

Allow the contents of the composer window to be piped to
a user-specified external filter, and the output of said
filter to replace the current contents. Certain editors
(e.g. Emacs) allow this and it's a useful facility when
you don't want to hand-format a large message. Of course
you can do the same with cut-and-paste, or saving and
reloading, but hey I can dream can't I?

Even better would be a hook for an external editor, but
I've mentioned this several times and there doesn't seem
to be a lot of interest. A filter would be next best thing.

poc