Add options for RefSeq and Merged caches when converting to MAF

[bkohrn]

It would be nice to be able to use RefSeq or Merged caches (VEP options --refseq and --merged), instead of being forced into just using the Ensembl cache. So far as I can tell, shouldn't be that hard; I'm just not a perl coder, so I don't know how to handle the inputs or what sort of error checking needs to be done before running (for example, probably want to throw an error if both --refseq and --merged are called).