Added ability to take input from stdin and output to stdout/stderr

This patch adds the ability for docco to take source code from the stdin and output to stdout or stderr:

```bash
echo -e 'test (test) ->\n  test' | bin/docco lib/docco.js -o stdout
```

This allows Docco to be part of a pipeline or used where the source code is not a file (e.g. generated client side JavaScript) and/or the output is not a file (e.g. delivered to the browser as a gzipped download)

Thanks