Enhancement: allow basic customisation

[assembledadam]

I think either allow customisation of the baked-in buttons or remove them entirely, leaving them up to the user to add (which gives them the opportunity to set custom configuration on the extension).

For instance, I don't want , underline, or H3 formatting. This should be a simple configuration, but right now it seems I have to fork the repo and modify the editor.js to add my own custom FormattingExtension.