Allow extending build with local-only files

The .user extension is in the .gitignore list, so Directory.props.user and Directory.targets.user can safely be created without risk of commiting to the repo. 

This allows to tweak local builds without changing commited files.

Author: kzu

src/Directory.Build.props

@@ -142,4 +142,5 @@
   </ItemGroup>
 
   <Import Project="Directory.props" Condition="Exists('Directory.props')"/>
+  <Import Project="Directory.props.user" Condition="Exists('Directory.props.user')" />
 </Project>

src/Directory.Build.targets

@@ -159,5 +159,6 @@
 
   <!-- Import before UsingTask because first to declare tasks wins -->
   <Import Project="Directory.targets" Condition="Exists('Directory.targets')"/>
+  <Import Project="Directory.targets.user" Condition="Exists('Directory.targets.user')" />
 
-</Project>
+</Project>