Header ID specifiers

Allows anchor IDs to be specified alongside block headers in Markdown Extra format, i.e.

```
[Go to the glossary](#glossary)
[Go to the Appendix](#apdx)

## Glossary {#glossary}

# Appendix # {#apdx}
```

Related issue: russross/blackfriday#17

(For comparison, the very different MultiMarkdown syntax for cross references is here: https://rawgithub.com/fletcher/human-markdown-reference/master/index.html)