Added option -C/--config to pixie cli: For specifying a configuration json file

[kc611]

As titled this PR adds the option -c/--config to pixie-cc which takes in the path to a json configuration file to specify the TranslationUnit and ExportConfiguration objects.

An example json configuration file is as follows:

{
  "translation_unit": [
    {
      "name": "llvm_foo_double_double",
      "source": "define void @\"_Z3fooPdS_\"(double* %\".1\", double* %\".2\", double* %\".out\")\n{\nentry:\n  %.3 = load double, double* %\".1\"\n  %.4 = load double, double* %\".2\"\n  %\"res\" = fadd double %.3, %.4\n  store double %\"res\", double* %\".out\"\n  ret void\n}\n"
    },
    {
      "name": "llvm_foo_float_float",
      "path": "llvm_foo_float_float.c",
      "extra_flags": []
    },
    {
      "name": "llvm_foo_int_int",
      "path": "foo_int_int.pyx",
      "extra_clang_flags": []
    }
  ],
  "export_config": {
      "symbols": [
        {
          "python_name": "foo",
          "symbol_name": "_Z3fooPdS_",
          "signature": "void(double*, double*, double*)"
        }
      ]
    }
}