refactor: plugin

docs: Settings

style: Allow multiple choice -> Use multiple rules

refactor: add random serial to avoid SEC_ERROR_REUSED_ISSUER_AND_SERI…

…AL error

feat: reserve x-forwarded-for by -M keepXFF

refactor: Error page

refactor: Remove duplicate rules

refactor: Help

Merge branch 'docs' of https://github.com/avwo/whistle into docs

refactor: Handle clientError