Add max complexity configuration from file

[rpdelaney]

Is your feature request related to a problem? Please describe.
The only documented method for configuring the max complexity is a command line switch. It is difficult to enforce a common standard in a shared codebase, or ensure CI is using the correct settings this way.

Describe the solution you'd like
A configuration option I can put in pyproject.toml to ensure the setting I desire is respected for all callers of flake8 in my repo.

Describe alternatives you've considered
Do nothing. Easiest, but it doesn't address the issue.

Additional context
N/A