readme: More troubleshooting guidance

Fix typo identified in issue jschuh#47

globals: Fix issue jschuh#50

draw: Add priming to DRAW_PURGE_LINE

start_end: Optional PRINT_START gcode before purge/print

layers: Simplify negative Cura layer handling

layers: Handle negative layers from Cura

See issues jschuh#48 and jschuh#27

start_end: Add start_end_park_y setting

globals: Cleanup print_min/print_max