add sanitizer option.

[howking]

undocumented markedjs/marked#468, but useful option.

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[howking]

I signed it!

[googlebot]

CLAs look good, thanks!

[howking]

This is sample for use.
https://github.com/howking/sanitize-element

<sanitize-element config='{
  "elements":["a","img"],
  "attributes":{"a":["href","title"],"img":["src"]},
  "protocols":{"a":{"href":["http","https"]}}
  }' sanitizer="{{sanitizer}}"></sanitize-element>

<marked-element markdown="[[markdown]]" sanitize sanitizer="[[sanitizer]]">

[stramel]

I'm good with allowing this pass-through but it should have tests supporting it and a better description of the property.

Would be good to get @e111077's input too.

[howking]

thanks !
I added tests and desc.

[e111077]

I gotta say; I really love this feature! Thanks a lot for the submission, @howking!

[stramel]

🎉 thanks @howking! Released with 2.3.0