Make setext header insertion work like rst-mode

Well, more like it, anyway.  I like how rst-mode lets me set up a
heading, change it, then hit a key to fully underline that heading as
appropriate.  This makes the markdown-mode setext-style heading
commands (C-c C-t t and C-c C-t s by default) work more like that.
The old transient mark behavior is now gone.

I would feel more studly if I had found a way to do this without using
(save-excursion) twice.