Add --fast flag to skip Markdown validation

[hukkin]

See #49 (comment)

Add a CLI flag for skipping Markdown validation.
I would name the flag --fast, --unsafe or --skip-validation.

[KyleKing]

Do you have a preference for which flag to add? I may have time to submit a PR by next week?

[hukkin]

I think --unsafe and --skip-validation are both alright.

I always thought if things are eventually stable for years, it may be possible to change the default value of this check to off. If that happens we need the opposite flag. Then we'd have either --safe or --no-skip-validation out of which --safe seems much better. But that'd translate to safe = true/false in TOML which with --unsafe would slightly break the rule where CLI opt naming directly corresponds with TOML attribute naming....

Would --no-validate and validate = false/true be horrible? 😄 Either that or just --unsafe and unsafe = true/false, and don't worry about the future. Whoever implements can decide 😄

[KyleKing]

I like --no-validate, but I would probably prefer to default to validate = true because plugins often break the AST (or maybe that's just me 😆 https://github.com/search?q=org%3AKyleKing+renders+to+different+HTML&type=issues)

[hukkin]

I would probably prefer to default to validate = true

Yeah right now the default must definitely not change. And if it ever changes it may be useful for experimental plugins to be able to keep the validation on, similar to how they can turn validation off via CHANGES_AST already.