ENH: Add a js extension in order to publish directly from the jupyter frontend

This is an enhancement to be added when the jupyter development is less subjected to API changes.

Previous version was [here](https://github.com/Pybonacci/ipy2wp/blob/master/ipy2wp/ipy2wp.js) and there was a problem on windows as [it wasn't able to detect the location of the templates](https://github.com/Pybonacci/ipy2wp/issues/7).
