templates: Add opentype feature picker

[m4rc1e]

This PR implements an OpenType feature picker in the html views. This feature was requested by @vv-monsalve (great idea btw).

Since the text boxes are also contenteditable, it's becoming a very nice playground to test out custom user text, similar to Impallari's tester.

[vv-monsalve]

Working beautifully! Thanks

[vv-monsalve]

Updating the report. It worked perfectly (without reported errors) at the proof option.

However, when using it with the diff option, the output folder was created, but it reported two different errors at two different run times:

• Using the --imgs option:

  ValueError: height and width must be > 0
  [2/2] _diffbrowsers diff -fb "/Users/v... ".*" --imgs --filter-styles "Regular"
  ninja: build stopped: subcommand failed.
  

• Without using --imgs option

  ValueError: height and width must be > 0
  ninja: build stopped: subcommand failed.
  

I'm attaching here a .zip file that includes

• The output folder (which includes the fonts to replicate the test)
• A .txt file where I copy/paste both full tracebacks.

qaPraise-L-01-diffOT.zip

[m4rc1e]

Thanks, I think this is related to #82 instead. i'll roll a fix now.

[vv-monsalve]

i'll roll a fix now.

Could the OT picker include the positional alternates (fina, init) like this one (second image)?

Posting it here since you'll work on the fix. Let me know if you prefer to track this in a new issue, and I'll open one.