Add max_warnings config option to `.flowconfig` file

# Proposal
There is currently a way to pass a `--max-warnings 0` argument to the `flow-cli`, but it is not possible to configure it in the `.flowconfig` file. I propose adding a `max_warnings` config option.

## Use case
I would like to keep the output of `flow` clean with 0 warnings and 0 errors. Currently, the CI does not fail on Warnings, so from time to time some warnings slip into the codebase. My current repository contains multiple services, where each service can call `flow` in their scripts. Adding a CLI argument will need to be repeated in multiple places in this case. All projects share a single `.flowconfig` file, so it would be great if the option can be configured in it.

See related issues:
* https://github.com/facebook/flow/issues/5009
* https://github.com/facebook/flow/issues/5612#issuecomment-357560355 (an alternative approach to elevate "Unused suppression comment." warnings to errors)