(TK-113) allow multiple comma-separated config files/dirs

This patch allows the specification of multiple config files and directories
in the config CLI argument like so:

lein run services --config dir1,dir2

Errors should be thrown in all cases one would expect if the contents of the configs were instead under a single directory.
This is a change to a public API, and there is code out there that definitely calls `load-config`.  May need to introduce a new function to handle the multiple-paths use case and have it wrap the old function or something.