Support more dbNSFP fields annotated by VEP

[a113n]

With reference to the PR in brentp/geneimpacts#12

[brentp]

sorry for the delay in answering.

@a113n thanks for these changes, but, I won't accept until the arey are optional. This adds a bunch of fields to the database even when those fields might not be present in the VCF.

The advantage of this over the existing method that @matthdsm mentions is that these are typed values whereas the current method sets all VEP extra's to TEXT.

I think a better strategy might be to add a --numerics argument that will accept a list of column names and force all of those to be sql.Float().