[TIMOB-18995] Windows: Provide ability to set capabilities on appx.manifest in tiapp.xml

The appx.manifest is where you set the capabilties and many other aspects of windows apps. Right now we use a templated version and write out one for the app during build. Some of that is populated with values from tiapp.xml. We need the ability for users to set the capabilities their app uses in their tiapp.xml and that gets copied into the appx.manifest. See the capabilities here: https://msdn.microsoft.com/en-us/library/windows/apps/hh464936.aspx Right now we're just hardcoding to a single "internetClient" capability.