Add 'Dlint verbose' mode as a flake8 formatter option

[mschwager]

Flake8 allows for custom formatters: Developing a Formatting Plugin for Flake8.

I think there's value in having an output mode where things are very dense, and there's one finding per line. This is the current formatter for flake8. It'd also be great if there was a verbose, multi-line mode that included additional information. E.g. the physical line that fired the rule and/or some amount of surrounding lines, a link to the rule's documentation, suggestions for a fix, etc.

I'm envisioning that we develop a flake8 custom formatter that keys off a rule's code and "pretty prints" some or all of the above information to the output buffer.