Add support for Axidraw layer name syntax #604
Description
This syntax enable controlling layer grouping, layer ordering, pen up/down position, pen down speed, as well as marking a layer as "do not plot" or enforcing a pause. Details here: https://wiki.evilmadscientist.com/AxiDraw_Layer_Control#Syntax
vpype write could emit those codes based on properties set by dedicated commands (axi --layer 1 -h 50 -d 30 -v bla).
Related to #471