Specify contract from live network

[chadoh]

We currently have this comment block in environments.toml:

# Coming Soon: 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

Let's start by implementing a simplified version of this. No spooning! Specifying a live contract to bind & import is still useful!

So this:

# Specify live contracts to bind & import in this project using the given name.
[development.contracts.eurc]
environment = "production"
address = "C..." # could also use `name = "eurc"`, which would fetch it from Registry

Later, if it becomes straightforward with the arrival of more Stellar data tooling, we can use #167 as our issue to track the implementation of the at-ledger-sequence line, a deceptively simple line which will require all manner of heroic data-fetching-&-transformation infrastructure.

[chadoh]

Must also support constructor_args and after_deploy like this:

[development.contracts.eurc]
environment = "production"
address = "C..."
constructor_args = """
--admin me --symbol LOL --initial-supply 1000000000000000000000000
"""
after_deploy = """
mint --to me --amount 1000000000000000000000000
mint --to alice --amount 1000000000000000000000000
mint --to bob --amount 1000000000000000000000000
"""