Draft mouse Y centromere

[eweitz]

The adds draft support for the mouse Y centromere in Ideogram.js graphics without cytogenetic banding information. It is a draft because:

• The centromere position noted in is 2,650,000 bp, but Ideogram requires a position of 9,650,000 bp to avoid a minor rendering issue.
• The centromere is not yet included in banded data.

This improves basic chromosomal accuracy of graphics for sex-specific mouse genes, like SRY.

[coveralls]

Coverage increased (+0.05%) to 90.625% when pulling ff57f18fae86cb9531c18afb9c4a4762a73ff05a on mouse-y-centromere into 1d551b461dc8aa399e843618c6e6a86857db262e on search-genes-and-paralogs.