Add a g:gtfo#terminals key for tmux

[bmedicke]

I prefer to split horizontally so having a key for customizing the tmux command would be great. It would also allow changing the working directory of a specific tmux pane without splitting at all, which might be nice.

I'm not sure how feasible that would currently be though, because splitting a pane and switching directories with tmux requires two sets of commands with the path in-between. And changing the path of an existing pane by sending keystrokes to it would get messy if there's already something running there.

[justinmk]

I'm thinking goT should open a horizontal pane. And the function previously served by goF and goT can be served by 1gof and 1got--that is, presence of {count} will open CWD. I never really liked goF/goT for that purpose anyway.