editorconfig: add max line

if we're going to have an editorconfig, might as well make it accurately
reflect the coding guide.

also set trim trailing whitespace to true.

.editorconfig

@@ -8,3 +8,8 @@ charset = utf-8
 insert_final_newline = true
 indent_style = space
 indent_size = 4
+max_line_length = 80
+trim_trailing_whitespace = true
+
+[.git/COMMIT*]
+max_line_length = 72