Refactor scene heading rule and specs

* Extract specs to standalone spec file.
* Make the scene heading openers white list (eg, 'EXT') case insensitive, per the spec
* Solve the esteban problem (close #8) where we want to be careful not to consider a line a scene heading just because it begins with the letters 'Est'. Solved by enforcing a space after a member of the scene opener whitelist. BUT not enforcing a space if the scene opener is a dot.
* Add a failing spec for scene numbers to encourage them to be implemented