(feat): Enable ability to pass custom capabilities to sauce browsers

Added the ability to pass custom options to saucelabs. 

Example use case - 

```
 customLaunchers: {
            sl_chrome: {
                base: 'SauceLabs',
                browserName: 'chrome',
                chromeOptions: {
                    args: ['--enable-gpu-benchmarking', '--start-maximized', '--enable-thread-composting']
                },
                'disable-popup-handler': true
            },
            sl_firefox: {
                base: 'SauceLabs',
                browserName: 'firefox',
                firefox_profile: fp.encodedSync(),
                'disable-popup-handler': true
            }
```
The firefox profile is initialized using [firefox-profile](http://github.com/axemclion/firefox-profile-js)

```
var FirefoxProfile = require('firefox-profile');
    var fp = new FirefoxProfile();
    fp.setPreference('dom.send_after_paint_to_content', true);
    fp.setPreference('dom.disable_open_during_load', false);
    fp.setPreference('dom.disable_open_during_load', false);
    fp.updatePreferences();
```