Feature: add .editorconfig

[rickvdbosch]

Description:

We should add an .editorconfig file for this repo, which also explicitly sets the whitespace character to be tabs.

For reasoning, please see:

• this comment by @CrypticEngima
• the article he links to
• and the root of it all, the Nobody talks about the real reason to use Tabs over Spaces post on Reddit

Steps:

1. Add .editorconfig file
2. Set tabs as the whitespace character