Extend sanitize

[kascote]

This PR extends sanitize_html to allow custom tags and attributes.

• Two new callbacks handle the checks
• The internal functions were normalized using the same parameters as the callbacks.
• Test added to check for the new functionality
• There are no changes to how the library worked before and do not require code changes.

An upcoming PR will handle the remove_contents option as observed on https://github.com/google/dart-neats/issues/101

[jonasfj]

lol, I just saw the old comment in https://github.com/google/dart-neats/issues/101#issuecomment-789717870 :rofl:

hmm, ...maybe, I'm should be more open to signatures like:

• bool allowTag(String tag), and,
• bool allowAttribute(String tag, String attribute, String value)

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Context: I use this library for sanitizing markdown on pub.dev, and I want that to be consistent with how it renders on github. But yeah, I can also see the value with allowing a few tweaks.. we did that for adding ugc attributes to links.