localhost JSON-RPC provider config

[RevCBH]

Changes to allow for selection of a JSON-RPC ethers provider, useful in testing against a local node.

Not sure how welcoming of the JSDOC type comments you are. Happy to remove them if they're unwanted.

Added a section to the README documenting the new configuration options.

[pinheadmz]

Untested but concept ACK, thank you!
I don't mind the jsdoc comments but I'd prefer not to add additional dependencies (@types/node) especially because this project otherwise isn't typescript. The README additions are great, thanks for that as well. The only implementation comment I have before testing locally is could we set the mainnet ENS contract as a default? And then if the user sets that option, it overrides ?