Add support for TargetTrivial

Example configuration:

    {
        "config":{
            "command":{
                "args":["{source}", "{target}", "{in_dir}"],
                "name":"shaders/make_shader_include.py"
            },
            "filename_ext":".hpp",
            "dependencies":[{"ref":"shaders/shader_include_template.xpp","rel":"misc"}]
        },
        "filename_exts":[".glsl"],
        "name":"targettrivial_default",
        "plugin":"targettrivial"
    }