Configurable validation for list of endpoints

Hey I added the skipping configuration we talked about in this issue -> https://github.com/striglia/pyramid_swagger/issues/31

Let me know what you think :)
General comment -- looks like your Travis run failed with flake8 issues. Make sure to run `tox -e pep8` and fix any issues before the next diff! https://travis-ci.org/striglia/pyramid_swagger/jobs/29287682