[github] change tab size to 4 spaces

The default tab size on GitHub is 8 spaces. This change decreases the
tab size to 4. I think this will make reviewing of code on GitHub
much easier.

This commit also introduces sensible defaults for editors which respect
the ".editorconfig" file standard.

This closes #1404.

.editorconfig

+root = true
+
+[*]
+indent_style = tab
+indent_size = 4
+charset = utf-8
+end_of_line = lf
+insert_final_newline = true