Support custom `consensus.json` files

[jdtzmn]

Problem

Algod allows for custom consensus.json files as long as they are located in the data directory, but getting a consensus.json file to the data directory requires the copyTo command and several docker up/downs. It would be nice if there was a CLI argument that allowed you to pass your own consensus.json file on ./sandbox up.

Related to #36.

Solution

Add a CLI argument to sandbox for copying a consensus.json file into sandbox.

Stretch goal: read the consensus file and only overwrite/insert consensus config that was changed; don't replace the entire file. This way users could specify "consensus overrides" instead of replacements, and would make the customization more future-proof.

Dependencies

None

Urgency

Not urgent. Would be a nice improvement.