Run a default hook if the specific hook doesn't exist.

We now support a default hook called (oddly enough) "default-hook".  If a specific hook is missing and the default hook exists, the default hook will be called with the name of the hook as the first argument.

This has been tested live with a tweaked [postgres charm](https://github.com/natefinch/pgcharm).  This charm was tested because it was the first one I found that was all symlinks to a single python script.  Removed all symlinks and made a very minor tweak to the python script to grab the first argument and use it as the hook name.  It works... dare I say... like a charm.
I'm in two minds over whether this should be in juju/charm/hooks -- I think it probably should be, but please make sure it's clearly documented.