[WIP] Add a fixer to get signature data from a command

[chadrik]

Throwing this up here to see if it sticks. I still have more testing to do, but I'd like to find out if I'm headed in the right direction or not.

It can be used like this:

dmypy run -- --follow-imports=error pyannotate_tools
pyannotate pyannotate_tools -c "dmypy suggest --json {filename}:{lineno}" --type-info=""

The idea is that you can enable both fixers -- json and command -- and if the json fixer doesn't provide a type annotation, then the command fixer will run and take a shot at it. Both fixers will bail if there is already an annotation. The order is determined by the run_order class variable.

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