Add 'variant' key to optional argument

[uliska]

Create a new key to the optional arguments: variant=. This should implement some kind of index lookup so one doesn't have to create individual commands for related glyphs.

An example could be the existing \twoBeamedQuavers which could make use of variants with different beam slopes.

Probably this shouldn't (only) be implemented as an integer lookup but through named key.

I think of command invocations like:

• ţwoBeamedQuavers[variant=01] -> different slopes
• twoBeamedNotes[variant=semiquaver] -> different number of beams

[uliska]

With image commands one could easily map the option's value to a file name suffix.
But what about glyphs? should be real lookup.

[uliska]

See also comments in #38