Add examples of extending an existing block

[aaronjorbin]

I would love to see an example that adds an additional sidebar control for 1) A specific block (maybe the paragraph block?) and 2) all blocks.

A quick and useful example could be storing "notes".

[aduth]

This is not yet (reasonably) possible. Related issue at https://github.com/WordPress/gutenberg/issues/2474 with some exploration of plans.