notmuch vim patches

[Franz Fellner]

> I'm starting to realize that I could default to using 'enter' to both
> open URI's and view attachments.  Any other ideas welcome.

- make some of the functions public so users can bind them to keys they
  want
- introduce show_[prev,next]_unread_msg, probably factor out
  "show_scroll_to_msg()" and implement prev/next msg with that?
- use proper buffers instead of Scratch buffers, so BufExplorer also
  shows notmuch-vim buffers
- folding of messages; add option to fold read messages by default.

I think I can implement the second point (if you not already did
that).
I also looked into folding, but my knowledge of all the possiblities is
not that good (manual folding markers look ugly, don't know if/how 'syntax'
will work; and IMHO the user's shortcuts for folding should still work).

Franz