Go Templating support in start / end gcode

[aligator]

Based on the work from #45
I think support for the Go templates would be really nice.
https://golangdocs.com/templates-in-golang

So for example the whole configuration and maybe some additional info such as max layer count could be passed to it and then it would be even possible to create conditional gcode templates (e.g. only if starting-temp > 0, set it.)

If possible: still allow the simple templates of #45 to not break the existing templates of the users.