Handle storyboards as XML files

When editing a file with <config-file> or <edit-config>, only a few extensions are recognised as XML, the rest are considered plists.

The CDVLaunchScreen.storyboard generated by cordova-ios is XML. To be able to edit it from config.xml, .storyboard files need to be recognised as XML.

Pull request: https://github.com/apache/cordova-common/pull/12

It may make sense to take this further, and either:

• reverse the test, and only consider .plist files to be plists, and the rest XML. Not sure if that could break something
• or add an attribute to <config-file> and <edit-config> to specify the type of file (XML or plist).

However for the purposes of modifying the storyboard the (very minor) change in the pull request is enough.