Support for short and long read alignments - read _bam / geom_read

[thackl]

Support read mapping visualization. Simple viz should be straight-forward ala geom_feat(position="pile") with some sensible defaults for large numbers of features. For reading .bam could borrow from bioconductor.

Might make sense to add some sort of cigar support to visualize errors - one issue would be, though that mismatches aren't explicitly encoded in bam (unless -X). Would need reference sequence to infer...

Down the line, would be cool to handle complex SVs from long reads - with supplementary alignments in style similar to linked exons.