Add support for dropElements

[0xedward]

Resolves #30

Use DOMPurify FORBID_CONTENTS configuration to emulate the behavior we want from dropElements

Example:

const sample = 'Some text <b><i>with</i></b> <blink><i>tags</i></blink>.'
let testSanitizer = new Sanitizer({blockElements: ['blink'], dropElements: ['b']})

testSanitizer.sanitizeFor('p', 'Some text <b><i>with</i></b> <blink><i>tags</i></blink>.') // returns "<p>​Some text <i>​tags​</i>​.</p>"​

DOMPurify.sanitize(sample, { FORBID_CONTENTS: [ "b" ], FORBID_TAGS: ['b', 'blink']}); // returns "<p>Some text  <i>tags</i>.</p>"

Same example with DOMPurify 2.3.1

There might be some subtle bugs when the config contains elements in both dropElements and blockElements (see cure53/DOMPurify#556 (comment)). I'll work on setting up the test infra and come up with sample inputs to test this

[mozfreddyb]

I'm not sure but maybe deduce this boolean flag easier from within the default-assignment logic above, don't you think?

[0xedward]

I'm not sure but maybe deduce this boolean flag easier from within the default-assignment logic above, don't you think?

I'm probably missing something obvious here. But do you have an example of how to merge the initialization of the booleans with the assignments above?

[mozfreddyb]

It seems we are checking if dropElements was set twice. Once in the dropElements = config.dropElements || [] line and then here. I'm wondering if those could be merged, to have the same semantic thing happen just once.

[0xedward]

Ah, I think I'm following! I'll try tackling this in #44, since that refactor will touch these lines as well (e.g. DEFAULT_DROP_ELEMENTS here will become a set)

[mozfreddyb]

I think this looks right, but at this point I really want tests 😂

[mozfreddyb]

It seems we are checking if dropElements was set twice. Once in the dropElements = config.dropElements || [] line and then here. I'm wondering if those could be merged, to have the same semantic thing happen just once.