mplexporter support

This is just a very preliminary WIP PR, with some basic matplotlib exportation support using the Jake's mplexporter library.
I am essentially opening this PR to keep a public track about the things I/we want to add (or not) and to discuss some other things I am finding whereas I am using the lib...

- [x] Support figure width and height
- [x] Basic line support.
- [ ] Complete line support.
- [x] Line plot example
- [x] Output file
- [ ] Output notebook
- [ ] Output server
- [x] This is not the end of the list :wink:

So, nothing to review yet, but you can take a look if you are interested, and make suggestions...