Use custom annotations

[julibeg]

It would be nice if we could pass a dict with custom annotations to the qm.manhattanplot function. For example, if I already know the IDs of my top hits and I would like to annotate them with the genes they are in, I could then pass a dict {snp_id: gene_name} to the function and it should replace the text annotations accordingly.

[julibeg]

Should be fairly straightforward to implement. Let me know if I should give it a go and I'll open a PR once I had time to have a look.