allow configurable metric_plugins_dir

This is mostly useful to make it easier to package GE as a deb or rpm, so people can install GE in system locations and still be able to disable or replace or enhance builtin metric plugins.

It allows the user to configure a list of plugin dirs. The system will load .py files from each one in turn, making it look like they were all part of the same python package. So plugins in a separate user dir will still be able to do "from . import Plugin" and get the right base class.