Auto discoverability of configuration settings

[guibou]

For now we have only one important option: --issuetracker-githubkey, but in a near future (when #33 will be merged) we'll have also a similar option for gitlab. We can also imagine as many option as it exists issue providers (such as gerrit, jira, ...). Also, I don't know how we'll handle "self-hosted" instance (of gitlab for example), but we may need a way to tell krank to look for some specific domain for issues.

All of that will need to be parametrized by the command line (or a configuration file). I'd like the CLI to give as many hint as possible so the user will have an experience free of documentation reading.

For example, if krank hit the API call limitation (see #17 for example), it may be interesting to return a hint set your github token. Same thing for private repositories.

If the checker finds an url such as https://xxxxx/foo/bar/issues/345, I'd like an hint saying something like:

"https://xxxxx.com/foo/bar/issues/345" may be an issue url, please add xxxxx.com in the list --gitlab-providers.