Adding 'o' key for Open/Close message window

So here is the thing. I consider Enter as a "too far" key (which is stressed by
my right-hand RSI). Since `o` key is unused for both message list and view
(both buffers are read-only), this patch adds Open and Close Message Window
mappings.

This is quick way of opening and closing (for consistency with Space/Enter)
messages. I like this very much and hope you will like it too.

If you are happy with the change, I will go ahead and add this mapping to the
wiki (list of keys page I suppose).

Vmail looks like my primary MUA now (happy Mutt user). I would like to help
implementing two missing things which I consider as "blockers" for me:

* Threads
* Vertical split (I have this nice super-wide panel and I want to see more)

Good stuff guys!