Add grid option, 'ticks'.

I need to use ticks. I appreciate that the grid serves the same purpose. And, moreover, your design choices reflect a sensibility much more refined than my own. I can't do without them (my PhD adviser insist, which is reason enough) and in my opinion they can be less intrusive that a grid in some cases. If you agree, here is a PR.

Since one would never want both ticks *and* a grid, it seems natural to add "ticks" alongside "whitegrid", "darkgrid", and "nogrid".