Support simple ${project_path} syntax in options

Simple ${project_path} can be convenient for per-project options like:

    "sublimeclang_options":
    [
      "-isystem", "/usr/include",
      "-I${project_path}/../../../../build/texk",
      "-I${project_path}/../../../../build/texk/web2c",
      "-I${project_path}/../..",
      "-I${project_path}/.."
    ]

It's simpler to specify than more versatile ${project_path:filename} syntax.