Fix linter npm-groovy-lint

[Jorenar]

I wanted to use npm-groovy-lint to check some Gradle files, but I was not getting any diagnostics.

Changes:

• removed filename field from handler's output (it was pointing to temporary file),
• removed --loglevel warning from default options (so by default we see linter's defaults),
• added --noserver as a default option (to avoid lingering process after closing Vim),
• appending rule name to diagnostic message (so user knows which rules to toggle/modify in linter's config),
• adjusted tests,
• updated docs.