Sphinx extension for defining options, add intersphinx

[Demo of option details](https://dl.dropboxusercontent.com/u/360865/mrjob_docs/guides/configs-all-runners.html)

[Demo of quick reference tables](https://dl.dropboxusercontent.com/u/360865/mrjob_docs/guides/configs-reference.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
```

I didn't try very hard to comment or organize the extension itself. It's about as readable as any given extension.