Add support for pull_request_target event

[OleksiyRudenko]

Please review.

Resolves #6

Project maintainers also may want to add a version tag when the PR is merged.

Suggested version is 1.1, as this change is not a breaking change and ensures backward compatibility.

[awmatheson]

Since I am no longer a GitHub employee I cannot merge this, I will try and find someone who is still there and might have access to merge this.