add gatsby plugin

[mathieudutour]

A 2 step installation for a gatsby app:

• npm install --save @prodo/core @prodo/babel-plugin @prodo/gatsby-plugin
• update the gastby config

Then you can use the model anywhere

[mathieudutour]

It would be really nice if the path to model file was also passed to babel plugin (#161) and used there as well.

Yep, that's one of the reason why I proposed that 😄