Add support for Tags in a charm

Adding Tags field in a charm, alongside with Categories. Tags will be moderated and used by GUI. 
Rather than making a new repo charm just for this, we could always reuse the
same charm and add some tags.

tbh though, I'm not sure that it's worth having a repo charm for either of those
things - we'd be better off if we just wrote the metadata.yaml file
from data in the test - it's only 150 bytes.

charms in the repo are useful reuse outside of the charm package,
but I don't see this being so. but... your decision, just saying.