Add label customization/options #243

These are probably different requests but here it goes.

1. Would it be possible to have the same options that we have for images? Something like:

labels: |
	name=my.label,value="my value",enable=true

The one I'm interested in is enable, but I figured it'd make sense to have it structured and not just

labels: |
	my.label="my value",enable=true

but I would be happy with that 🙂

2. Would it be possible to add expressions to labels? Or some way to add info from the action?

labels: |
	version={{version}
	branch={{branch}}

The same output that we get for steps.meta.outputs.version that automatically has the version no matter what kind of even triggered it would also be nice.

Thanks!