Add conda recipe

[jonmmease]

This PR adds a conda recipe for plotly_express. It uses Jinja2 templating to automatically pull in the version, description, and dependencies from the setup.py file so it should be pretty low maintenance.

I uploaded the output of this recipe to the plotly anaconda channel at https://anaconda.org/plotly/plotly_express.

To build the conda package

$ conda install conda-build
$ conda build recipe/  -c plotly

To upload to the plotly anaconda channel

$ conda install anaconda-client
$ anaconda login
$ anaconda upload ... # The path that was output by conda build above

[nicolaskruchten]

reminds me that we need to add patsy directly as well to close #18