This is a PR to fix ethers-io/ethers-cli#6. The --registry option allows a user to select a custom ens deployment, which is particularly usefull for chains where there is no official deployment.

Example:

node packages/cli/lib/bin/ethers-ens.js \
    --rpc https://bellecour.iex.ec \
    --registry 0x5f5B93fca68c9C79318d1F3868A354EE67D8c006 \
    lookup core.v5.iexec.eth

Note: Some custom have no controller/registrar/legacyRegistrar, so I had to make the getter form these safe (return null is nothing found) and add checks in the different plugins.