Support additional capability options

[travisbrown]

Thanks for this project!

I've found that with the default configuration of recent Selenium versions (Selenium server version: 3.141.59, revision: e82be7d358) I seem to have to provide additional desiredCapabilities options in order to get non-null sessionId values in responses. Specifically I've had to add the following:

{ "goog:chromeOptions": { "w3c": false }}

I'm currently using a modified version of the project with a start_session that allows the user to pass in arbitrary extra JSON configuration, but this could also be done in a more restricted way. Is this something you'd be willing to consider a PR to add?