Add .editorconfig

In order to guarantee this plugins code does not violate diferent IDEs/editors different coding styles, a .editorconfig file should be created.
This file should unify the conding style for different IDEs/editors.

More information:

- https://editorconfig.org/

Reference:
- https://github.com/orca-services/cakephp-heartbeat/blob/cakephp-3.x/.editorconfig