Add the ability to test the connection without printing

[ryall]

I would like to be able to test that the printer is online, without actually printing anything.

[receiptline]

Thanks for using ReceiptIO!

Try running it with the -q option.

$ receiptio -d 192.168.192.168 -p escpos -q

If the printer is online, it will return 'online' (exit code: 100).

[ryall]

Awesome, thank you!