ENH volcano plots

[BorisMuzellec]

What does your PR implement? Be specific.

This PR implements a method in DeseqStats to make volcano plots.

Example:

counts = load_example_data()
metadata = load_example_data('metadata')

dds = DeseqDataSet(counts=counts, metadata=metadata)
dds.deseq2()

ds = DeseqStats(dds)
ds.summary()

ds.plot_volcano(LFC_threshold = .25, pval_threshold = 0.05)

[umarteauowkin]

I approve as it is a good first draft ! In an ideal world, all the fixed kwargs should be modifiable, but in that case I think people would just recopy the code !