Simple API for defining a block

[mtias]

We need a simple tool to let us quickly define a block (root element, custom attributes) to speed up the prototyping.

[aduth]

Should this issue encompass the final API for registering blocks, or is it merely for iterating on the UI prototype with a basic block structure (which already exists to an extent)?

[mtias]

This was for the UI only, so let's close it.

[aduth]

Should we create another issue for discussing requirements for a final block API?

[mtias]

Yes, feel free to start one.

[aduth]

See #104.