Support --strict flag that fails with non-zero exit code for warnings

[kdawgwilk]

To ensure docs don't get merged with invalid links or syntax that fails to parse it would be great to add support for a --strict flag that fails the mix docs task with a non-zero exit code so that way we can create a job in CI to indicate an issue with doc generation before it merges.

[wojtekmach]

Duplicate of #1294. :) we don’t plan to work on it ourselves but if someone will send a PR we'll gladly accept it!