Add .editorconfig file (#133)

Adds an [editorconfig file](https://editorconfig.org) which allows
editors to default to the correct indentation and general file
formatting used by code in this repo.

.editorconfig

@@ -0,0 +1,7 @@
+root = true
+
+[*]
+indent_style = space
+indent_size = 2
+trim_trailing_whitespace = true
+insert_final_newline = true