fix(setup.py): add ext subpackage

Author: tomas-fp

setup.py

@@ -14,7 +14,7 @@
     url='https://github.com/tomas-fp/python-jsonpath-ng',
     license='Apache 2.0',
     long_description=io.open('README.rst', encoding='utf-8').read(),
-    packages=['jsonpath_ng', 'jsonpath_ng.bin'],
+    packages=['jsonpath_ng', 'jsonpath_ng.bin', 'jsonpath_ng.ext'],
     entry_points={
         'console_scripts': [
             'jsonpath.py=jsonpath_ng.bin.jsonpath:entry_point'