Add 'ligature codes' to access alternates

[Dave Crossland]

Add 'secret codes' that use highly unusual strings (like i_alternate_glyph to access i.ss01) to access the alts.

Alternatively, a post-build pyftfeafreeze process to make a 2nd set of files with a different family name, with all the "aalt" glyphs "feature frozen".

(Either approach will be needed to use these alts in Docs/Slides)

[Dave Crossland]

@Fonthausen please take a look at this for the Romans

[Jacques Le Bailly]

@davelab6 You mean different than the existing ss01 and ss02 features ?

...with all the "aalt" glyphs "feature frozen". Where the alts become the defaults ?

[Eli Heuer]

I looked at my notes and these are the alternates I know Radio-Canada wanted:

Also, adding info about using the alts to the README was requested.