Support mapping to different Ensembl versions

[LTLA]

Currently the Ensembl version is hard-coded at my choice of AH number, but there's no reason to limit users to that. The idea here is to support ensembl = "99" to instruct the mapper to map gene symbols to Ensembl 99 identifiers.

Conversely, the datasets provided with Ensembl annotation should have the option to include symbols in their rowData.

We pay the cost of pulling in ensembldb and AnnotationHub anyway, so we might as well get maximum mileage out of it.