Add editorconfig.

.editorconfig

+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+[*.py]
+indent_style = space
+indent_size = 4
+
+[*.html]
+file_type_emacs = jinja2
+indent_style = space
+indent_size = 2