Helper functions/API for plugins

[siliconfeces]

To reduce copy/paste between plugins, it might be prudent to prepare some some helper functions as an "internal API". handle_tag_yaml is IMO a great candidate, after a rename.

Negatives can be that these functions will have to stay around for a very long time to keep compatibility.

[Difegue]

I don't mind adding some more helper functions for plugin dev, although they should probably have their own namespace 🤔

Docs should probably be slightly amended to mention those too, currently the rule for plugins is "internal functions are subject to change at any time, use w/ caution"