tools: add .editorconfig

This helps editors (see http://editorconfig.org/#download) to
apply the right indentation and encoding rules to files by
default.

PR-URL: https://github.com/nodejs/node/pull/2993


Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>