Allow external config file

Example to use: (assuming the config is in the project root)

```xml
  <Import Project="..\packages\NArrange.PreBuild.0.2.9.1\build\NArrange.PreBuild.targets" Condition="Exists('..\packages\NArrange.PreBuild.0.2.9.1\build\NArrange.PreBuild.targets')" />
  <PropertyGroup>
    <NArrangePreBuildConfig>..\MyProject.NArrange.Config.xml</NArrangePreBuildConfig>
  </PropertyGroup>
```