Add long output to Python aggregate_signals() method

[chinandrew]

Closes #214

Summary of changes

• Add argument for "long" output format which concatenate signals together. Argument is output_format instead of the R package's format since Python uses format for string formatting.
• Add tests and update documentation

Should probably refactor these methods into their own file in a later PR (e.g. wrangle.py, similar to R file) since the covidcast file is getting large.