loam-cli/environments.toml: Allow specifying live contracts to spoon into local network

[chadoh]

This is going to require collaboration with people working on indexing.

In our originally-proposed environments.toml spec, we said this would look like this:

# Specify live contracts to bind & import in this project using the given name.
# During initialization, these contracts will also be "spooned" into the development network,
# meaning that their data will match the live network at the given sequence number.
[development.contracts.eurc]
environment = "production"
address = "C..."
at-ledger-sequence = 50153603

We can potentially use snapshotting to accomplish this.