feat: Customize WebDriver config in custom launcher

[tykus160]

Example of custom launcher with this PR:

module.exports = function(config) {
  const firefoxOptions = {
    'moz:firefoxOptions': {
      prefs: {
        'media.eme.enabled': true,
        'media.gmp-manager.updateEnabled': true,
      },
    },
  };

  config.set({
    customLaunchers: {
      'FirefoxEME': {
        base: 'Firefox',
        config: firefoxOptions,
      },
      'FirefoxHeadlessEME': {
        base: 'FirefoxHeadless',
        config: firefoxOptions,
      },
    },
  });
};

[joeyparrish]

I think this is really useful. But for shaka-player, I think those settings you quote in the PR description should be the defaults for Firefox.

[joeyparrish]

Thanks!

[joeyparrish]

Could you please document the config field in the Configuration section of the README?

Also, could you please add this to the EXTRA_WEBDRIVER_SPECS for Firefox launchers by default?

    'moz:firefoxOptions': {
      prefs: {
        'media.eme.enabled': true,
        'media.gmp-manager.updateEnabled': true,
      },
    },

[tykus160]

Done. I hope provided example is good enough.

[joeyparrish]

Thank you!