use config.get directly instead of probot's getConfig call

[jetersen]

Could be improved even more by using: https://github.com/probot/octokit-plugin-config/blob/a613c0efd2873fb64a3a87f79365ec1951fd29ad/src/index.ts#L13 directly, as we would be able to specify a branch to get the config from. Actually this function is even better as it will attempt to use .github repo without using _extends

Originally posted by @jetersen in #365 (comment)

[mkurz]

+1
Would be nice to directly reference a file within an organization's .github repository with the config-name config.

[jetersen]

@mkurz no config-name is one thing.
_extends by default will extend to .github repo on org.
See: https://github.com/probot/octokit-plugin-config/blob/a613c0efd2873fb64a3a87f79365ec1951fd29ad/src/util/get-config-files.ts#L42-L46

you can of course also specify that your _extends if you want to use another repo or another file. See: #935 (comment)
I don't think _extends should take an branch argument.

_extends also support file names: See https://github.com/probot/octokit-plugin-config/blob/a613c0efd2873fb64a3a87f79365ec1951fd29ad/src/util/extends-to-get-content-params.ts#L8-L15