Doc with markdown header is not formatted correctly

[sethladd]

See this file: https://github.com/localvoid/timezone/blob/master/lib/timezone.dart

Specifically:

/// # TimeZone library
///
/// Time zone database and time zone aware DateTime.
library timezone;

It's rendered as:

Expected:

The word TimeZone should be a header (H1?)

[sethladd]

The Dart language spec used to have a subset of markdown for what you can use inside of doc comments. The latest version of the spec (second edition) no longer specifies what goes into doc comments.

There's probably enough doc comments that use markdown that we need to support it.

[sethladd]

This is what the previous dartdoc used: https://pub.dartlang.org/packages/markdown

[sethladd]

The cleanupDocs method does some markdown formatting, but misses headers.

[sethladd]

This should be fixed.