Enable custom rulesets for the lintMarkdown builtin

[eytobo]

• Add a new parameter --rulesets <ruleset path> to the lintMarkdown linter implementation, which allows passing the file with custom rules for Markdownlint.

[dsalaza4]

@eytobo Is this done?

[etobo-dev]

Yes, it's done. I didn't close the issue because I couldn't find an option for it.