Minor problem in Variant.__init__

[iskandr]

The "fast" path for initializing a Variant object didn't convert the start parameter to an int. Also added support for creating the variant's associated genome from the string name of a reference.

[tavinathanson]

If we're letting a Genome be an int release or a str reference_name, we might as well do the same in load_vcf so that @timodonnell's release_num code doesn't break?

Offline discussion: the load_vcf genome argument can just be passed through to Variant?

[tavinathanson]

Only question I have is: seems like infer_genome is basically duplicated logic? https://github.com/hammerlab/varcode/blob/fix-variant-start-and-string-reference/varcode/variant.py#L114 vs. https://github.com/hammerlab/varcode/blob/fix-variant-start-and-string-reference/varcode/vcf.py#L440?