Add support for ligatures

[Naereen]

• I want the "latin" ligatures, using the unicode caracters ff fi fl ffi ffl ſt st and 🙰 as input in the template, but using a proper ligature markup in the metadata.json dict, to effectively allow my font to use the handwritten character for ff or st (or ch or whatever) instead of just two "f".
• if it's easy to do, I will do the same for maths and code characters, like FiraCode.
• being smart about the "ligature caret" is mandatory (see how it works in FiraCode).

[Naereen]

Look for ligature here: https://fontforge.github.io/python.html#Glyph

[Naereen]

And https://fontforge.github.io/meta/guide-page-3.html

[Naereen]

See 7e50c6c, 28fd7d0, 63577ac and b17eb26 for a first start...

[Naereen]

OK for the data and the template, I will finish this later.

[Naereen]

Starting to work on parsing the scanned template.

[Naereen]

I'm almost there!

[Naereen]

https://github.com/ToxicFrog/Ligaturizer and https://stackoverflow.com/questions/10902593/fontforge-scripting-how-to-add-ligatures-for-a-glyph

[Naereen]

[Naereen]

It's too bad, everything is here to do it but it's quite hard.

[Naereen]

This script is much simpler, https://github.com/IlyaSkriblovsky/dv-code-font/blob/master/conv.py, I could use it to understand everything FontForge needs to have...