support --skip-no-token

[suzuki-shunsuke]

In some situation, the GitHub Access Token isn't exposed to the environment variable for the security.

For example, on Drone Secrets are not exposed to pull requests by default.

https://docs.drone.io/secret/repository/

Secrets are not exposed to pull requests by default.
This prevents a bad actor from sending a pull request and attempting to expose your secrets.

github-comment requires the GitHub Access Token, so it fails to run github-comment post and github-comment exec.

This issue proposes to add the option --skip-no-token to prevent the failure.

If the GitHub Access Token is set, this option is ignored.

If the GitHub Access Token isn't set, this option works like --dry-run.

[suzuki-shunsuke]

--skip-no-token is a little long, so we want a good alias.

[suzuki-shunsuke]

• add the configuration option skip_no_token
• test
• update README