debug mode for spacefinder

- small fix for custom selector checks (see line 30 - all tests should be from the insertion point, e.g. the top of the paragraph)
- added debug argument that annotates paragraphs with classes/attributes to expose debug information (to be used with the bookmarklet [here](https://gist.github.com/mattosborn/c6cc61a69bb049b45446))

Screenshot
---------------
![image](https://cloud.githubusercontent.com/assets/960919/5377148/5a97095a-8076-11e4-9f55-2fb6af79f1b0.png)

// @ironsidevsquincy 
yea would be good to dig deeper. didn't really fancy refactoring all of this though and tbh the output seems to be helpful enough! 

if this gets used and the users find the output is too vague then i'm happy to do a refactor. hoping this will be enough though.