Support ABC files in Music21Dataset

[salu133445]

The current implementation of Music21Dataset does not support ABC files. The problem here is that an ABC file might contain multiple songs (i.e., a tunebook). We should follow the implementations of ABCFolderDataset to extend self.raw_filenames into a list of (filename, start, end) tuples to keep track of the start/end line of each song in an ABC file (and simply set start and end to None for other file types).