Add a .editorconfig file

[jeffwidman]

This is an easy one if someone's looking for a way to get involved with open source: #2040 (comment)

add an .editorconfig file to specify newlines at end of file, spaces rather than tabs, etc.

[lixiaoqity]

It is a good idea to have an .editorconfig file to keep all the file consistent with the style. I will pull a request to add this file.