Support structural variants

Currently small variants are supported as gk.Variant(chrom, position, position, ref, alt, reference_genome).

There are large variants in the human genome (i.e. thousands of bps):
* Copy number variants
* Deletions
* Insertions
* Translocations (this might be the trickiest since it's inter-chromosomal)
* Inversions on the same chromosome

It would be great to support these types of variants and be able to call genome.dna(interval) - but I realize this would be very difficult in terms of coordinates.

requested by @helen-zhu