Provide kerning pairs for \lilyText and \lilyNumbers

[uliska]

Printing numbers and (dynamic) characters with \lilyText pretty much cramps the letters, and there should be some kerning applied. This should be implemented through kerning pairs and a gsub(pair, a..'\hspace{kernvalue*option}'..b) replacement. Pairs should be storable for different fonts, where only a basic set has to be complete while other fonts only have to specify differing values.