Here is an attempt to support the geo parameter and fix #14. It works fine, but the code looks ugly. I'm not sure that I like going down this path for this package.

Here is the issue: the Covidcast API has multiple ways to specify arguments.

• data_source and signals or signal={source}:{signal}[;...]
• geo_type and geo_values or geo={geo_type}:{geo_values}[;...]

This translates into ugly interfaces for the functions in this package

• most arguments become Optional (e.g. look at the call method I modified)
• lots of validation boilerplate to see which of the above arguments were used
• argument order doesn't matter which encourages confusion

So I'm not sure what to do.