Make compile_commands.json paths also absolute.

[mcourteaux]

This fixes a ton of issues for me. My usual workflow is as follows:

mkdir build
cd build
cmake ..
cd ..
ln -s build/compile_commands.json .

Such that I have sym-linked compile_commands.json file in my project root. This file is automatically synced by the build folder, which is perfect. Color_coded however, does not correctly interpret the paths, as they are all relative to the "working_directory" entry of the compile_commands.json file. My pull request fixes this.