Support for update/edit or delete of annotations

[stevehawley]

Addresses issue #14

[VitaliyKotov]

@stevehawley Hi, when this PR is supposed to be merged?

[stevehawley]

Hi @VitaliyKotov, that would be up to the maintainer

[danilofuchs]

@Secretmapper please take a look

[Secretmapper]

Hi! Thaks for the PR!

This functionality is already possible with Custom Content - namely, renderContent can have the buttons for delete/submit, and also be an editor, similar to the threaded comments example.

I think keeping the core package lean is best as this can be done in userspace (and in fact would have to be redone if the user wants different styles), however, it might be a good idea to have an example (and also make it easier to write custom content). I made an issue #36 tracking it so I can add it in the future (PRs welcome as well)