[FIX] implement pre-ordering of ESCPOS commands

The proposed change fixes an issue with ESCPOS commands. The command used for 'size', ESC !, is a batch command that also sets font, emphasis and underline properties. Because the code sets the latter three properties by individual commands, we have to ensure that these (ESC -, ESC E, ESC M) are issued after any ESC !.
The proposed solution adds a '_order' attribute to the commands so that they can be sorted.