Asynchronous commands

[zxvcv]

There is need to implementing asynchronous commands to control the printer form the PC even if there is print task ongoing.

Before each command form PC should be send one char to to determine the type of task. [type][data]

Type should be interpreted as "unsigned char"

Code for types (as number) of command:

• 1 - normal command
• 0 - asynchronous command (should be invoke right after recieve), (prioritized)