Adds editorConfig.

Fixes #5491

.editorconfig

@@ -0,0 +1,22 @@
+# http://EditorConfig.org
+# https://github.com/editorconfig/editorconfig/wiki/EditorConfig-Properties
+
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+indent_size = 2
+indent_style = space
+insert_final_newline = true
+trim_trailing_whitespace = true
+tab_width = 2
+
+[**.rb]
+max_line_length = 80
+
+[**.js, **.coffee]
+max_line_length = 120
+
+[*.md]
+trim_trailing_whitespace = false