Pre-up, post-up, pre-down, and post-down hooks

These are programs that, if they exist, will run before or after the
syncronization/removal is run. Three use cases caused this:

1. The thoughtbot dotfiles will run a vundle installation set of
commands after intitial synchronization.

2. I changed the location of `.bash_history` to `.bash/history` and
wanted to move `.bash_history` to `.bash/history` after up to preserve
existing history.

3. Moving from an existing old-style custom install script to `rcup`
might require some cleanup; this happened in practice, and required a
simple script.
Add -kK for rcup and rcdn

The hooks can be skipped using `-K`, if needed and they can be forced
with the `-k` flag.