Allow adding plugins temporarily without modifying plugins/CMakeLists.txt

Useful to avoid issues when switching between branches that modify
plugins/CMakeLists.txt

plugins/CMakeLists.custom.txt

@@ -0,0 +1,3 @@
+# You can add custom plugins here to avoid touching plugins/CMakeLists.txt,
+# This can be useful if you've made modifications to that file and try to
+# switch between branches that have also made modifications to it.

plugins/CMakeLists.txt

@@ -180,3 +180,5 @@ OPTION(BUILD_SKELETON "Build the skeleton plugin." OFF)
 if(BUILD_SKELETON)
     add_subdirectory(skeleton)
 endif()
+
+INCLUDE(CMakeLists.custom.txt)