Add support for user installation access tokens

[marcusrbrown]

I've used this action in several workflows in an organization account to generate tokens from my private apps to access any repos owned by the account. I want to move that exact use case to my personal and service user accounts, so I need an access token scoped to all repositories accessible by the user installation.

A couple of details need to be sorted out before this is ready:

• Handle the case where someone passes both organization and user input.
• Update tests to distinguish user vs. org test data.

Thanks for any feedback.