Add object specific comments before gcode blocks

[shocksofmighty]

Here is a proof-of-concept that I just put together in a few hours.

https://pastebin.com/dvfuuna0

It has the basics and in this case I am just skipping the process 'P2'. Missing a retraction, but that could be configurable in settings.

[shocksofmighty]

Here is what I have worked up so far. Totally bare bones and only works with S3D process comment lines. It is functional, but not really fleshed out yet.
https://github.com/paukstelis/Octoprint-Cancelobject

I've chosen to call it cancelobject instead of cancelprocess just in case other slicers add object specific comment lines.

[shocksofmighty]

Just a bump. Both Slic3r and Cura have versions of this now (at least in developmental versions).