Use the issuetracker's New functions to make a validation on the authentication type.

If an unsupported authentication type for the given issue tracker is provided, return an error "unsupported authentication type for : "

Supported authentication types by issue tracker:
Github - none, apitoken
Gitlab - none, apitoken
pivotaltracker - none, apitoken
r

followup from #149

	// makeExample replaces all ${linter} placeholders to a bound linter name.

The code below does not use ${linter} anywhere.
I could remove this, but would also like to avoid doing several unrelated things on the same PR, so keeping it as an issue to avoid forgetting about it.

The current logo is a bit... bland?

None of the maintainers are designers, so we picked something that was simple and functional.
Maybe now is a good time to revamp the look a bit and work on a more distinct branding.

Here are some values that the new logo should convey:

• clear
• distinct
• objective
• analytical (heh)

I don't mind if the logo is a bit playful and colorful, but I wo