Add ability to pass additional options for chrome/gecko drivers

[Kirguir]

Motivation

At the moment there is no possibility to pass command line flags (or "switches") for a chrome driver. This is needed to access the media device in tests. For example, such as '--use-fake-device-for-media-stream', '--use-fake-ui-for-media-stream'.

Proposed Solution

Pass additional options through the environment variable 'GECKODRIVER_OPTIONS', as well as for the server part ('GECKODRIVER_ARGS').

[alexcrichton]

This should in theory be supported via CHROMEDRIVER_ARGS (like geckodriver), does that work for you?

[Kirguir]

CHROMEDRIVER_ARGS its arguments for spawn binary. switches => Capabilities, they hardcoded at moment. if add flags here, it works.

[alexcrichton]

Ah ok, makes sense! Sounds like a good feature to me to add and a PR should do the trick!