Add EditorConfig file

This makes any many expectations on file formatting explicit.
Based on the ring library's exemplar.

Author: Sander Maijers

.editorconfig

@@ -0,0 +1,13 @@
+root = true
+
+[*]
+# Visual Studio 2017 does this wrong
+# https://developercommunity.visualstudio.com/content/problem/22922/editorconfig-support-interprets-charset-utf-8-as-u.html
+# charset = utf-8
+end_of_line = lf
+indent_style = space
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+[*.{rs,toml}]
+indent_size = 4