Create basic .editorconfig file.

As proposed and accepted in #3715. I have found these files to be very helpful (as long as developers are encouraged to install the plugin).
We can discuss how this can be generalized and extended.

Reference on what is possible: http://editorconfig.org/
Per the [docs](http://editorconfig.org/#file-format-details), `tab_width` defaults to the value of `indent_size`.