Accept default capabilities

[raelgc]

This will enable default capabilities available for all drivers: https://www.w3.org/TR/webdriver1/#capabilities.

Usage example (included in README file):

    "testEnvironment": "jest-environment-webdriver",
    "testEnvironmentOptions": {
      browser: 'firefox',
      capabilities: {
        browserName: 'Some user agent',
        acceptInsecureCerts: true
      }
    }

[raelgc]

Any chance this get merged?

[raelgc]

Ping.