Add site-specific binaries to $PATH

Since different sites may neeed different scripts, we can now put them
in $RCDIR/site/bin/ so as to make them available to a given site as
though they're builtins without affecting this repo or the system.
Load site-specific tmux options

On different sites, there may be different projects or different
layouts/bindibgs, and this lets us do that easily, by taking advantage
of our nice $RCDIR/bin directory. Neato.
Store/restore vim sessions per git project

Each git project now stores the latest session of vim on every write and
successful exit. If vim is them started without any arguments, that
latest session will be restored. Otherwise, a blank session will be
started. This is to prevent bloat as well as to allow opening files
directly (if sessions load automatically, it would otherwise override
any files specified).
Add "kill" shortcut (^B,^K) to tmux