tmux: additional session configs, split them out

Easy to load one-by-one with source-file.
vim: Finally getting around to YouCompleteMe

I'm normally leery of plugins this expansive and with this much
extension code, but it seems pretty solid and SuperTab has never served
me very well (to be fair I probably never configured it well, but it
*has* too much config to think about, and that's part of the problem...
hg: df abbrev. Used so much with git I can't stand the incongruity
hg: Switch to the core shelve extension