Use pane_id, the pane index is not stable

This also changes the formatting string to include the window name and
pane title, which should make is easier to pick the right pane.

Fixes https://github.com/epeli/slimux/issues/10
Added SlimuxSendKeys function

with some wrapper commands for easy mapping. This is similar to
SlimuxSendCommand, but it is a bit more low level as it doesn't insert carriage
return(Enter) automatically and enables sending special keys to the pane(such as
ctrl+c for SIGINT).

Also modified SlimuxShellLast so it will prompt for a command if none was sent
before.
Merge remote-tracking branch 'tarruda/send-keys'