* Change name to isLabeled for re-use with "exempt"
* Add args for exempt labels
* Skip operation if labeled exempt
* Only check for exempt label if defined
Since the exempt label parameter is not required, we don't want to both
checking for it if it was not defined.
* Don't reduce operationsLeft if exempt
This variable just tracks the number of operations we're using to avoid
rate limiting. Since the exempt check wouldn't result in a rate-limited
request, we don't need to reduce the variable.
* Update the action.yml with new exempt values
* Update README with example of exempt labels
* Remove redundant defaults
