Enable Flake8 validation in Hound CI

Will check all pull requests in the project to make sure they don't
violate PEP 8 style (except the E501 for long lines because I think
it makes code hard to read).

.flake8

@@ -0,0 +1,2 @@
+[flake8]
+ignore = E501

.hound.yml

@@ -0,0 +1,3 @@
+flake8:
+    enabled: true
+    config_file: .flake8