Add exportGcode method for Klipper Users (non-WebSerial printing)

[nkymut]

Thank you for creating this amazing library.

In my specific use case, I'm running Klipper on a RaspberryPi, which prevents me from connecting a serial connection directly with my 3D printer. To work around this issue, I've added an exportGcode method which allows for the downloading of gcode files so it can be sent to the printer via Mainsail.

I've tested these changes in my local setup (Ender-3 Pro + Klipper + Mainsail) and confirmed that almost all of the provided sample files can be successfully printed. This addition should help other users who have a similar setup to mine.

[nkymut]

As minor issues associated with the gcode output,
the following have been observed while printing:

1. The predicted completion time does not settle, instead it keeps increasing on Klipper screen.
2. The print bed does not cool down after printing is complete (I think it is necessary to add sets of print completion commands such as print-bed cooldown).

[bsubbaraman]

Thanks for the kind words, and for the contribution! I've been away, but will get around to merging this pull request next week

[msurguy]

+1 on the export to gcode button!