Sphinx extension for defining options, add intersphinx

[Live demo](https://dl.dropboxusercontent.com/u/360865/mrjob_docs/index.html)

The intersphinx extension adds links to external sources (in our case, the Python stdlib docs). So if we type ```:py:mod:`shlex````, it will link to the actual docs instead of just being bold and monospaced.

As for the new extension, you define an option like this:

```rst
.. mrjob-opt::
    :config: base_tmp_dir
    :switch: --base-tmp-dir
    :type: :ref:`path <data-type-path>`
    :set: all
    :default: value of :py:func:`tempfile.gettempdir`

    Path to put local temp dirs inside.
```

You generate the table for a set of options (in the above, that would be `all`) like this:

```rst
.. mrjob-optlist: all
```