Add a new log option to createAppAuth(options), then replace console.warn with state.log.warn

[gr2m]

We are trying to avoid any direct calls to console, so that consumers of the Octokit libraries can use their own logging tool. The way we do that is a log option that is compatible with the global console object. It should at least set { debug, info, warn, error }. By default, { debug, info } are no-ops, the other two default to console.warn and console.error respectively. Compare https://github.com/octokit/core.js#logging / https://github.com/octokit/core.js/blob/37437bcf632ce32ba25663e575de96a844b802b2/src/index.ts#L111-L119

We introduced console.warn statements in #164. These should be replaced with what ever is passed as options.log.warn to the createAppAuth method.

I'd happily accept a pull request and will provide guidance if you are interested in contributing. The pull request should add a test (or adjust an existing one), it should implement the feature, and add the new option to the documentation in README.md

[romanbalayan]

Hi @gr2m,
I opened a PR for this. I'm still quite unsure how to go about testing the log methods - debug, info, and error. Currently they are not covered on the test cases.

[gr2m]

thanks @romanbalayan, great work! resolved via #190