Require user to provide `toolchain.json` config

The user is now required to provide their 32 and 64bit mingw or
mingw-w64 flavored toolchain configuration information via a
`toolchain.json` file in the root directory. The config file
must look similar to:

  {
    "x32": {
      "path": [
        "C:/DevKit-x32-4.7.3/bin",
        "C:/DevKit-x32-4.7.3/mingw/bin"
      ]
    },

    "x64": {
      "path": [
        "C:/Apps/DevTools/msys/bin",
        "C:/Apps/DevTools/mingw/bin"
      ]
    }
  }

Currently the "x32" object is required and the "x64" object is
optional.