Attributes Extension

I realize this pull request may be a bit premature, but I wanted to put it out there for people to have a look and give feedback. (Otherwise just merge the typos.)

This proposal was [started on talk.commonmark.org](http://talk.commonmark.org/t/consistent-attribute-syntax/272) and any substantial feedback on the attribute syntax should go there.

There is also a more general [discussion about the standardization of extensions to 'core markdown'](http://talk.commonmark.org/t/optional-syntax/515). For example, should this really be in the same document as the core spec, or should each extension get its own spec document?