[Feature Request]: Make grizzly serve work with multiple directories

[markmarkmarkthebest]

Feature Request

In my case I use grizzly to control grafana instances in different environments. They all have the same dashboards but the datasources are different and managed in different directories and there is one common directory which holds all dashboards (without the datasources). So ideally to serve grizzly for some env I would need to run:

grr serve -f <common_dir> -f <env_dir>

But seems like grr serve currently supports using providers only from a single directory.