[tmux/en] [WIP] Massive re-write to tmux tutorial

This is a work-in-progress, though I believe that it is nearly complete.

I wanted to show a lot more than I had previously, though this may have gotten a little out of hand for a casual glance in comparison to the previous version. I feel that a majority of the available tools/features that tmux has to offer should be noticeable during a quick skim, and the bulk of the information is still digestible over a short period of time.

I may do some further editing, though I'd love to hear other's opinions on the rewrite!