Add the settings to create mapping.

I did some work related to #117 and #22.

It's the first step to have a better configuration of the index settings. 
On my side, I needed this functionality to define custom analysers for some fields and I didn't want to manually create for all my ES environments the mapping and the settings. To avoid duplication, I created in my Mongoose models a static method to create indexes where I have my settings but a better approach would be to have the settings declared somehow in the Schema description. 

I also modified the README and added a test case (even if, according to me, the tests do not cover a lot except that it's not failing).

Let me know if you have a better approach and then I can improve it