OverrideUserAgent AppendUserAgent support

[anotherapj]

I'm looking for support to add the OverrideUserAgent / AppendUserAgent that was introduced on the InAppBrowser plugin. There are pull requests open, it would be great if someone merges the pull request into master. This is required for my project that requires the web pages to identify the custom user agent and perform some UI modifications based on that.

Refer the open pull requests #72 for IOS & #71 for Android.