Build JavaScript files during python package build (isso-comments#697)

Make package builds via setuptools automatically trigger building of
merged & minified JS assets.

This fixes installing isso from source when using "pip install" as long
as both the python and node requirements are met. It also ensures that
setup.py commands like "build" or "bdist_wheel" create archives that
contain generated JS code.

setup.py

@@ -1,11 +1,25 @@
 #!/usr/bin/env python3
 # -*- encoding: utf-8 -*-
 
+import os
+import setuptools.command.build_py
+import subprocess
+
 from setuptools import setup, find_packages
 
 requires = ['itsdangerous', 'Jinja2', 'misaka>=2.0,<3.0', 'html5lib',
             'werkzeug>=1.0', 'bleach', 'Flask-Caching>=1.9', 'Flask']
 
+
+class NpmBuildCommand(setuptools.command.build_py.build_py):
+    """Prefix Python build with node-based asset build"""
+
+    def run(self):
+        if 'TOX_ENV' not in os.environ:
+            subprocess.check_call(['make', 'init', 'js'])
+        setuptools.command.build_py.build_py.run(self)
+
+
 setup(
     name='isso',
     version='0.12.5',
@@ -34,5 +48,8 @@
     entry_points={
         'console_scripts':
             ['isso = isso:main'],
-    }
+    },
+    cmdclass={
+        'build_py': NpmBuildCommand,
+    },
 )