Include .editorconfig

I saw the [following commit](https://github.com/twitter/bootstrap/commit/32761b33036eb3a0e186d310b56877b1c994ac89) and thought you need an .editorconfig file for Bootstrap.

 *[EditorConfig](http://editorconfig.org/) helps developers define and maintain consistent coding styles between different editors and IDEs.*


EditorConfig plugins are currently available for: Code::Blocks, Emacs, Geany, Gedit, jEdit, Notepad++, Sublime Text 2, TextMate, Vim & Visual Studio.

I think I have the settings correct based on the [code guide](https://github.com/mdo/code-guide):

`indent_style = space
indent_size = 2
end_of_line = lf
charset = utf-8
trim_trailing_whitespace = true
insert_final_newline = true`