Problem: Limit of one taco format per project

Currently there is no way to use multiple taco formats in a single project (probably) [due to the way taco_format must be defined within the configuration](https://github.com/simble-app/tacos/blob/master/lib/tacos.ex#L70).

As a user, I want to store fixtures of different formats within my app to test data parsing for JSON, YAML, and CSV.  CSV support would be snazzy too.
