Ability to add ligature-substitutions via FontBuilder interface

[Vftdan]

Currently FontBuilder only has a method addFeatureVariations that only adds single-substitutions.
This issue proposes to create a FontBuilder method of a form:

    def addLigatureSubstitutions(self, mapping):
        from .otlLib.builder import buildLigatureSubstSubtable
        subtable = buildLigatureSubstSubtable(mapping)
        ...

that adds ligature substitutions to the GSUB table and adds all required metadata to other tables; or a more general method that can perform this task. Like with addFeatureVariations, the actual logic may be added as a function of varLib, feaLib or another fontTools module.