Add custom_checks/* to package_data

setup.py

@@ -24,7 +24,7 @@ setup(
     packages=['validata_core'],
     include_package_data=True,
     package_data={
-        'validata_core': ['schemas.toml', 'validata_spec.json'],
+        'validata_core': ['schemas.toml', 'validata_spec.json', 'custom_checks/*'],
     },
     install_requires=[
         'goodtables',