Implementing alignment

[hedgerwang]

@ marijn’s suggestion is right.

I was able to get this working by using custom schema for the paragraph node (and other similar block nodes).

• Source Code https://github.com/chanzuckerberg/czi-prosemirror/blob/master/src/ParagraphNodeSpec.js#L41
• DEMO: http://cdn.summitlearning.org/assets/czi_prosemirror_0_0_1_b_index.html