Create FileTypeFinder helper class

Typed from memory online, so there are likely bugs in this version.
forgot the "add one" part
forgot a comma

Someone figure out a better way to make lists, without needing commas at the end! Maybe call it YML, or something...
Load file_type_finder.rb in engine
add file type to Github Blob Onebox
oops. no extension in require
Ruby and JavaScript don't need special "lang"
whitespace
harmonize variable names and add a warning

Future devs will really be helped by this warning!
Merge pull request #254 from kohenkatz/code-highlight-by-filename

Code highlight by filename