Provide means to pass additional desired capabilities to remote driver

[goodspark]

I'm currently using bok-choy for some tests. I'd like the following:

1. Run tests on different operating systems, versions, etc through a testing service, like SauceLabs or BrowserStack.
2. Pass additional metadata about my test runs that those services use to tag/filter runs in reports.

Normally these are set through desired_capabilities passed to the remote WebDriver, not just the tags field that Sauce Labs uses.

Currently, desired_capabilities is derived from _capabilities_dict. It manually maps environment variables to specific, hardcoded capability keys.

Instead of adding more env vars that must be manually mapped and maintained, how about exposing an other_caps parameter to the browser call that will get merged with the derived capabilities?

To maintain existing env-var-mapping behavior, the derived capabilities would override any passed in capabilities from the user.

Thoughts?

Looking at the code, I believe the change is fairly simple and I can do it myself. Would like to brainstorm it with you all first.