Add shuffle rhythms in alphaTex

[cheap-glitch]

Description

Currently there is no way in alphatex to indicate a shuffle rhythm at the beginning of a score like in GP. It would be useful to have at least the "triplet shuffle" (two eighth notes are equivalent to a quarter and an eighth in a triplet) and the "dotted shuffle" (two eighths are equal to a dotted eighth and a sixteenth) as those are the most prevalent types of shuffles.

[Danielku15]

Yes, this feature is missing. But please for the future try to fill out all sections provided by the template even if you answer them with something like "no known workaround". This keeps consistency among al feature requests.

Also I always appreciate if people suggest/describe how they would like to see a feature in alphaTab.

[Danielku15]

Triplet feel can not be specified in alphaTex.