Automatically create anchors for section headers

If I write something like this:

```
## Foo
```

and I want to link to it later, I have to do this:

```
<a href="#foo">
## Foo
</a>
```

or the equivalent. GitHub and rustbook generate these kinds of links automatically, which keeps source files cleaner. Example: https://github.com/rust-lang/rust/#quick-start
