Include mplexporter submodule within matplotlylib package

[jakevdp]

I think it makes sense to have a script which includes the mplexporter submodule in the matplotlylib source (see the mpld3 Makefile for an example of this)

The way it's set-up now, you'd have to remember to manually install/update both mplexporter and matplotlylib. Combining them will make installation much smoother.