Retro Thread viewer!

Hi, Pazz! You've made a quite impressive MUA! Thanks!
I'm comming from Mutt and found in Alot a lot of features that I missed in Mutt. ;)

But I found the current thread viewer a bit problematic with big threads, and saw that I'm not the only one:
https://github.com/pazz/alot/issues/694
https://github.com/pazz/alot/issues/695
So I made a "Retro Thread" (RT) viewer, that somehow replicates Mutt behavior: with a small viewer for the thread tree and a big message viewer. 

The code is a big mess and not finished, so I'm not sure I should be sumitting a pull request, but it seems usable here and faster in big threads than the current one.
I tried to place most of the code related with the RT in a sepate file, to don't "corrupt" your code. =P
But had to add a command (in `alot/commands/search.py`) to enter this "alternative" viewer, and a binding group (in `alot/commands/__init__.py`) for the bindings of the new viewer.
(Besides that there are 2 typo-corrections in this pull)

To use the code, you'll probably need to add a few binding in the config file.
In **[[search]]** add:

    <some key> = RTselect

And than the new bindings group, something like:

    [[RTthread]]
		l = move next
		h = move previous
		j = move down
		k = move up
		z = move switch
		r = reply

Best regards!