Retaining feature popup fields for MappedFeatureSource

@dasmoth cc @mdrasmus

When feature tracks are lifted-over using a mapping chain, some fields in the feature pop-up which are populated by default ([for example] (https://github.com/dasmoth/dalliance/blob/master/js/vcf.js#L146), in `vcfParser` for a VCF file) are lost.

This PR propagates the  `getDefatulFIPs` function of underlying feature source of `MappedFeatureSource` to ensure that such feature pop-up information are retained. 

This seems to be the desired behavior from my use cases, and thought others might find it useful. :)