Add an editorconfig to support indentation and whitespace style

editorconfig is a standard for a file specifying whitespace preferences
in a codebase. Most editors and IDEs support plugins that will detect
this file and apply the styles therein while you edit code.

.editorconfig

@@ -0,0 +1,17 @@
+# EditorConfig: http://EditorConfig.org
+
+# Top-most EditorConfig file
+root = true
+
+# Rules for JavaScript files:
+
+[*.js]
+# 2 space indentation
+indent_style = space
+indent_size = 2
+# No trailing spaces
+trim_trailing_whitespace = true
+# Unix-style newlines
+end_of_line = lf
+# Newline ending every file
+insert_final_newline = true