12-factorize application a little

snikket_web can now be fully configured via the environment alone,
no extra files needed. It is still supported to inject a python
file to generate environment variables though, which may be
useful for generating and reloading a secret key.

build-requirements.txt

@@ -1,2 +1,3 @@
 pyscss~=1.3
 mypy
+python-dotenv~=0.15