Add --with-xvfb-options to accept options for XvfbWrapper

I'm running nosetests inside a Docker container which does not play well with SELinux.  So I found it necessary to invoke Xvfb with `-extension SELinux`.  The following patch allows you to pass arguments into the xvfbwrapper.Xvfb class.  Unfortunately, the kwargs argument to __init__ is handled like so
```python
for key, value in kwargs.items():
    self.xvfb_cmd = self.xvfb_cmd + ['-%s' % key, value]
```
which precludes passing in things like "+extension BLAH" or the ability to pass in the same option more than once.  Nevertheless, the proposed patch does handle most simple cases.