fix terminal stuff

TERM shouldn't be set in my scripts - it is provided by the vt. Duh!

If xterm, kill smcup and rmcup.
better hot keys for tmux

C-a 0123456789[] for window switching
C-a hjkl for pane switching
C-a s for session picking?
That's how it is set now. Could be improved.
add markdown detection

.md files were detected as modula-2.
Who uses modula-2 these days?
I don't.
tmux: mouse mode?

This is a controversial option. Sometimes I want it, sometimes not.
TODO: invent a key to toggle all mouse actions.