Plugins from GH should be loadable from subdirs

[pauldix]

Currently, the loading mechanism for plugins from Github only works for plugins two directories deep (e.g. examples/wal_pugin/wal_plugin.py. We should support an arbitrary number of nesting levels. Further, the github plugin spec should have the file, just like the local plugin dir does. This is instead of the current setup which assumes a plugin file of the same name as the parent directory with a .py extension.