Bump spec to 0.3.2, optional section attributes

[mixonic]

Bump the Mobiledoc spec to 0.3.2.

This adds an optional "section attributes" item to markup sections and list sections.

• The attributes are encoded as a list of key/value pairs, for example: ['data-md-text-align', 'center']
• Only data-md-text-align is permitted by the spec. An alternative would be to permit arbitrary attribute values, but claim the data-md- namespace and only restrict values added there.
• The attributes could alternatively been located in a new top-level property of the spec, but I chose to simply add them to each section instead. It seems fine for an initial pass, if every section will have an attribute we might want to encode them in a different way.
• I've added this to markup and list sections. Alternatives would be a) just markup sections b) markup sections and list item sections.

Any thoughts? I've got another PR incoming that polishes @rondale-sc's prior work to implement this in Mobiledoc-kit.

This is an extraction/edit from WIP #670

/cc @ZeeJab @kevinansfield @rondale-sc

[YoranBrondsema]

I continued work on this as we'd like to offer text alignment in Sutori. I made a new PR but started off from yours.

Only data-md-text-align is permitted by the spec. An alternative would be to permit arbitrary attribute values, but claim the data-md- namespace and only restrict values added there.

I'd be in favor of only permitting data-md-text-align attributes for now, and then see what use cases come up. Then eventually extending it or permitting any attribute. I agree that all attributes set by Mobiledoc should be prefixed with data-md-.

I've added this to markup and list sections. Alternatives would be a) just markup sections b) markup sections and list item sections.

I tried all three and from a UX perspective, I think you picked the right one.

[mixonic]

@YoranBrondsema I think the only real blocker on this is the renderers themselves needing to support the spec. Want to schedule a pairing session to try and wrap it up? I'll send an email.