Remove errexit for update function.

 - This seemed like a good idea a while ago, but it turns out not to be
   so great in practice and makes troubleshooting errors harder.
Update brew topic.

 - Simplify how I keep track of installed kegs.
 - Only keep track of top-level items (no dependency libraries) by using `brew leaves`.
 - Keep track of taps as well as kegs.
Update ssh config.
Fix the node topic messing up my path.

 - Add new function called path-ensure that just makes sure a directory
   exists in your path using path-append, but doesn't move it if it is
   already there.