Save arbitrary genomes as entities in the DB

[tavinathanson]

For example:

pyensembl add-genome --name "mouse_81" --gtf-path <URL> --transcript-fasta-path <URL>
pyensembl install "mouse_81"

rather than:

pyensembl install --gtf-path <URL> --transcript-fasta-path <URL>

From @iskander:

we want to even extend it to:

pyensembl add-genome-sequence "mouse" --transcript-fasta-path <URL>
pyensembl add-genome --name "mouse_81" --gtf-path <URL> --genome-sequence "mouse"

This would allow for easy integration into topiary, via:

topiary --genome "mouse_81"

This PR should, I think, also autopopulate the current Ensembl release mappings in the DB.