Support custom `consensus.json` files

## 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.