feature request - support specifying timeout for pdf generation

`timeout` can be passed here as argument to `communicate` to avoid waiting forever. 

Example: bad media URLs that cause long/indefinite wait times. 

https://github.com/JazzCore/python-pdfkit/blob/fb86d338a3706e6e0ede09170b9fa58fb740ec1e/pdfkit/pdfkit.py#L190

