Implement fully custom rich copy-paste processing

[thibaudcolas]

Currently, the filters operate on Draft.js content as provided by the built-in Draft.js copy-paste processing. This has the advantage of being more light-touch, but greatly reduces how much can be adjusted to either preserve more content, or filter more intelligently.

I think this is worth revisiting with the release of Draft.js v0.11, which has a new rich text copy-paste processing implementation of questionable benefit.

This is blocked by: #284

This would resolve: #12, #14, #212, facebookarchive/draft-js#166

Relevant: facebookarchive/draft-js#721, thibaudcolas/draftjs-conductor#269

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Alternative: formatPastedText from Draft.js v0.11.7, however that means being able to do more filtering only, not more "preserving".