Select all errors, warnings, and flakes.

This makes sure we're taken care of incase more stuff ever gets added
to the default ignores for flake8.
diff --git a/tox.ini b/tox.ini
index 5ff0877..4248db3 100644
--- a/tox.ini
+++ b/tox.ini
@@ -41,3 +41,4 @@
 
 [flake8]
 exclude = .tox,*.egg
+select = E,W,F