Add default value for the the github-token input

[EricCrosson]

Since you are likely to be rate-limited if you use this action without a GitHub API token, default the action's github-token input to the token from the github context^1.

Closes #3

[Hoverbear]

Heck ya, it works! You're awesome!

[Hoverbear]

I think we can remove the guidance from the README about the token now!