Ability to select by layer

[thatch]

Similar to object-<n>-use, I'd like something layer-<name>-use to choose based on layer. This is probably an input-only param, and doesn't need to be recorded in the output file.

Nice to have would be a way to have a default of off, then include just the one layer I want, rather than disabling them individually. I will provide a sample dxf that includes layers.