API Enhancement: Allow geneset creation with IDs other than MyGene IDs

[ravila4]

Currently, MyGeneset's user_genesets endpoint expects a list of gene IDs that are MyGene primary IDs: https://github.com/biothings/mygeneset.info/blob/master/src/web/handlers/api.py#L80-L86. This is because the frontend app only uses mygene ids for lookup.

However, if we want to expand the usefulness of the API, we can enable this endpoint to accept any kind of ids. The utilities module mygene_lookup already supports creating a geneset from any type of ID (it's being used by the various data plugins).

It would just be a matter of changing the _query_mygene function in api.py to take a parameter to allow specifying the type of input id for the input gene list. It could be an id_type parameter to POST requests, accepting a string or comma-separated list of input id types.