Add 'config' input to specify path to codecov.yml

[rhysd]

In my case, codecov.yml is not put at root of repository. When I used Travis CI, YAML config file could be specified with -y option of downloaded bash script. But this action does not provide the same functionality.

This PR adds config input to provide the feature equivalent to the -y option.

[thomasrockhu]

Unfortunately, -y flag is being deprecated (https://github.com/codecov/codecov-bash/blob/master/codecov#L929). Thank you for your PR

[rhysd]

ah, ok. I'll move the file under .github directory.