This is basically a major rewrite of the .ycm_extra_conf.py file that
ycm uses to determine what compile commands it should use for a file.
But the basic purpose is for it to use the generated flags in
build/compile_commands.json that is generated by cmake for as many files
as possible.
Before it always used the hardcoded default flags which would not always
work for all files.
Now it gets the proper flags for all files I tested.
In the src and tests folder the flags are already generated by cmake
In the include folder it tries to use the flags of the corresponding
source files in the src folder
