[LDCad] Rebrickable LDCad PBG export

[Orion Pobursky]

Tested on 45678 Spike set, works rather well!
- desciption -> description (I guess that description will be updated to include set name?)
- Would be nice to append -1 to set number if no -x is entered.

Thanks for testing. I'll implement those corrections/suggestions.

Now that it works as I intend it to, it's a matter of code polish and moving it to the main cms.