add editorconfig file to unify coding style

  [EditorConfig](http://editorconfig.org/) is a means to help developers keep a consistent coding
  style across IDEs/editors. This way we can all  speak and edit in the same way.

  @bluesmoon If theres not everything you require for your work on boomerang please tell me
  and I'll append it. 
According to what is in the documents "smart tab indents" are not covered by editorconfig. You'll still have to do this by hand but it also isn't affected by editorconfig itself.