Add support for Appium locate strategies.

[garg3133]

This PR adds support for missing Appium locate strategies used to locate elements in native apps.

[vaibhavsingh97]

We can improve this by using Factory Pattern here.

class Locator {
  create() {
    console.log('Locator create called');
  }
}

class AppiumLocator extends Locator {
  create() {
    console.log('AppiumLocator create called');
  }
}

class NightwatchLocator {
  static create() {
    if (isAppiumClient()) {
      return new AppiumLocator().create()
    } else {
      return new Locator().create()
    }
  }
}

// We can directly use this 
NightwatchLocator.create()

[garg3133]

I'm not sure about this, because the main reason for creating a separate class for AppiumLocator was to avoid passing a boolean flag (isAppiumClient) to create() method. /cc: @beatfactor

[beatfactor]

Well this would be ok, but the main issue is to check isAppiumClient() inside the locator class.

[garg3133]

Please check 4fd9c47.