Call scene headings scene headings not slugs
Add a scene number spec and many pending describes

Basically just copied the http://fountain.io/syntax navigation as
placeholders into the spec, which is probably going to need to be broken
into separate files at some point.
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
Merge pull request #14 from maxjacobson/organize-and-add-specs

Organize and add specs