sort source file list
Add an extension block to CMake
Render tests cmakelist before custom template

Otherwise you cannot overwrite custom CMakeLists.txt
Add support for include directories (fix #7)
Sort all the source lists, not only target.arm

This extends the idea of a8fc2f4ae to all source lists.
Make generate_source_list really return a list

It is way better when the API really does what it says it does.
Sort before returning from generate_source_list