Request: Accept cram input to toil-exome

`cram` is increasingly popular and is easily convertible to `bam` (and vice versa).

The 1000 Genomes Project has lots of exome data, accessible through ftp, but in `cram` format. 
For example:
ftp://ftp.1000genomes.ebi.ac.uk/vol1/ftp/data_collections/1000_genomes_project/data/CHS/HG00472/exome_alignment

Since the toil workflows already accept ftp inputs, it save users a download-and-convert.
