Add option to not fail on only Warnings

[thebeline]

There is an option to prevent golangci-lint from blocking a PR (by setting --issues-exit-code 0), but there currently is no way to configure the linter to only block on errors, and not on warnings (which seems like reasonable behavior, considering the two options are differentiated and supported by both golangci-lint and GitHub.

This PR adds the allow-warnings: true option, which, in the event that golangci-lint returns an exit code of 1, causes golangci-lint-action to check the output, and if there are no ::error notifications, return an exit code of 0, allowing the PR or Push to proceed.

This allows annotating the code with warnings/suggestions that are non-blocking, but blocking in the event that there are errors.

It should be noted that this requires golangci-lint to be configured to exit with 1 on "Issues". If golangci-lint does not exit with 1, then the action assumes that it is supposed to pass, and does not modify the exit code. That is to say it either exits with 0 if golangci-lint exits with 0, or exits with N, where N is a positive exit code not equal to 1 (some other error).

The way this should be configured to exit "conditionally" is to set --issues-exit-code 1, and in severity: set your default-severity: and rules: accordingly.

This is tested and works accordingly.

[thebeline]

Going to be replaced.

[thebeline]

Replaced By: #207