Add support for multiple experimental signup slugs

This allows configuring several multiple experimental signup slugs by setting `ENV['EXPERIMENTAL_SIGNUP_SLUG']` as comma-separated strings.

The behaviour for all these slugs will be the same (redirect to signup.heroku.com when any experimental slug is found) but having several of them will allow us to track several signup pages separately.
Merge pull request #107 from heroku/raul-support-multiple-experimental-signup-slugs

Add support for multiple experimental signup slugs