Add setting to use the current session/window

This adds two optional settings:
g:tslime_always_current_session
g:tslime_always_current_session
These will let tslime default to the current (the one with vim in it) session/window, I feel this setting is justified. I think it's a common case to wan't tslime to run in the current window.

A (regarding to me) bugfix is also included, that ensures that the current pane can't be selected, which also makes it faster to use.

I can split these into two PR:s if thats wanted, but I feel they are somewhat related and belongs in the same PR.