Support Kramdown footnote definitions and links.

[rhdunn]

The following are incorrectly syntax highlighted:

[^test]: This is a test.
[^ME]:   [Middle English](https://en.wikipedia.org/wiki/Middle_English). Wikipedia.

The footnote is correctly highlighted, but:

1. only the first word of the text is highlighted (e.g. This in This is a test.);
2. the links are not highlighted as links.

I think the:

[name]: url

style markup should not be applied if the name starts with a ^. This would then keep the definition in normal text colour and allow links and other markup to be correctly highlighted.

[cirosantilli]

I'll mark this as a feature request since it is a Markdown extension. Implementation under an option is welcome.

[cirosantilli]

Same for the links: testing[^test] (a test).