Rustdoc: Frontmatter Support

This is the initial PR to add frontmatter support. Note that it
currently only supports strings (and booleans) as values. Lists,
integers and such can be added in future PRs.

This doesn't integrate the frontmatter into the rest of rustdoc as I
wanted to get this out there.

```markdown
---
foo: "bar"
description: "123"
title: "983
Foobar
444"
---
```


/cc @brson