config-file element should support a src attribute

[wildabeast]

Right now a config-file element looks like

<config-file target="res/values/strings.xml" parent="/resources">
    <string name="app_picker_name">Applications</string>
</config-file>

would like to add support for sourcing this from a file:

<config-file targe="res/values/strings.xml" src="plugin-res/strings.xml" />

where plugin-res/strings.xml contains:

<resources>
    <string name="app_picker_name">Applications</string>
</resources>

So that if both a plugin and the target application have the same xml resource file, they get merged.