Support markdown header links

[seaneagan]

Should be able to link to headers in markdown files (readme.md and changelog.md). Also should be able to click on headers to link to them. For example:

https://pub.dartlang.org/packages/test

contains a relative link to #platform-selector-syntax. So when you click on it the browser url becomes:

https://pub.dartlang.org/packages/test#platform-selector-syntax

but it doesn't actually navigate you to that header.

And also you can't click on the header itself to get a link. (Maybe it's not implemented in the markdown package?) Both of these things work on github though:

https://github.com/dart-lang/test#platform-selector-syntax