ClosedCurve and ExteriorRegion support

[kburns]

Thanks for this great package! I'm interested in using AAA-LS to solve problems on exterior regions around smooth closed curves, and would like to use continuous AAA to get poles for the LS problem. I'm currently hitting a couple snags:

1. Using approximate on a closed curve (from ComplexRegions.jl) stops on missing dist for ClosedCurves. Should I be defining this myself?
2. Using approximate on an ExteriorRegion stops on ExteriorRegion has no field boundary.

Any suggestions? I don't have much experience with Julia yet but I'm happy to try to help with any necessary additions. Thanks again!