Features from Kramdown

Hi!

I quickly read throught the Specification and was missing Header IDs and Inline Attribute Lists (and also Typographic Symbols and Tables). Were they considered for the Spec?

Note that both are annotated:

Difference to Standard Markdown

Both are extensions similar to Markdown Extra.

There is a long list of possible extensions to core markdown
that might be useful. Those should be discussed at some point, but
this project is focused on codifying core markdown features.