Add <mark> to DEFAULT_ALLOWED_TAGS

[dogweather]

This is a safe tag and a common use case for sanitize in Rails: It's a semantic tag that's useful when showing search results. Search results can contain user input. Hence, best practice is to run them through sanitize.

E.g., in my code:

sanitize(search_hit_html, tags: %w[mark])

• https://developer.mozilla.org/en-US/docs/Web/HTML/Element/mark

[flavorjones]

This seems reasonable to me.

At some point we should audit the differences between Loofah and R::H::S, because Loofah has allowed this tag since v2.0.0 in 2014.

[dogweather]

Maybe a PR to DRY up the code?

[flavorjones]

@dogweather I want to audit the differences before writing code, I started doing this for attribute in #136 and hope to finish unifying Loofah and R::H::S as much as possible for the next big release.