more flexible annotation features

One of the issues I'm noticing is that since GK is structured around what features are exposed from annotations (eg, UTRs had to be added manually as a table), it limits general querying of stuff.

If possible, it would be nice if a user could specify which features to parse and build into tables to query. One of the issues with this is that GK fixes up missing/broken hierarchical information (gene->transcript->exon|intron) where as a general one won't.