Support docformatter as an additional formatter

[madig]

Environment data

• VS Code version: 1.22.1
• Extension version (available under the Extensions sidebar): 2018.3.1
• OS and version: Win 10 x64
• Python version (& distribution if applicable, e.g. Anaconda): Official 3.6.5 x64
• Type of virtual environment used: N/A

Actual behavior

Yapf sadly does not format docstrings.

Expected behavior

Something formats docstrings so I don't have to.

Please support docformatter as an additional formatter to run after/before the actual code formatter.

[brettcannon]

I'm sorry to say that due to the added complexity that would be required to support multiple formatters at once just for docformatter, we won't be adding support for this. Maybe you can see if yapf would add support, or maybe Black?