Snippets fr multiple locations

[nikhojman]

Is your feature request related to a problem? Please describe.
We're hosting a SaaS platform using JupyterLab 3.X and v2.1.0 of this extension. We're unable to collect snippets from multiple locations - in our case, the snippets needs to be a combination of a number of default ones as well snippets created by the user.

Describe the solution you'd like
To be able to collect snippets from multiple different locations.

[sntgluca]

Seems similar to #121 and #207 :) but with a more generic twist.

Personally, I think this plugin has a tremendous potential as a collaboration tool.
Having the possibility to store plugins in different locations would make people able to share their solutions, with all the benefits of the case...

I am thinking that it could be addressed by configuring a search list either in the advanced settings or via environmental variables, and having the plugin load the content from the list...
How does it sound?