bin/gftools-family-html-snippet.py Improvements

Migrated from https://github.com/googlefonts/fontbakery/issues/1447

@davelab6 originally said:

bin/fontbakery-family-html-snippet.py can be improved in a few ways.

Script:

- [ ] Add an arg to post to jsbin using https://jsbin.com/help/api

HTML:

- [ ] Add an input for testing with various subsets; allow free text entry since there are undocumented subsets that shouldn't be exposed in the code but can be entered
- [ ] Add a toggle for staging servers

I took a quick pass at this this morning but gave up as I need to move on to other stuff :(

https://jsbin.com/fonuwik/edit?html,js,output

See old comments at https://github.com/googlefonts/fontbakery/issues/1447 for further discussion.