Map DNA to offset cDNA/RNA variants

[mattdoug604]

It'd be useful to be able to map DNA variants to cDNA or RNA variants when the mutation occurs in an intron or UTR, instead of just the other way around.

For example, this works:

>>> ens100 = EnsemblRelease("homo_sapiens", release=100)
>>> ens100.to_dna("ENST00000307078:c.2237+3G>A")[0].to_string()
'17:g.65535623G>A'

It'd be nice if this also worked:

>>> ens100 = EnsemblRelease("homo_sapiens", release=100)
>>> ens100.to_cdna("17:g.65535623G>A")[0].to_string()
'ENST00000307078:c.2237+3G>A'