Allow lib users to pass capabilities to Client::new

[phrohdoh]

Capabilities allow, among other things, selecting which browser to use.

See selenium/python/desired_capabilities for a list of pre-defined capabilities we could expose.

[kilork]

+1

Good to have possibility to start headless session, I think capabilities is a way to do this, right?

[jonhoo]

I'm actually not sure. In Firefox, I think the way to do this is by requesting an extra argument: https://developer.mozilla.org/en-US/Firefox/Headless_mode. But I'm not sure that counts as a capability...