Add a unit test for an address with no utxos

chronik's response shape needs special handling by app devs, since chronik.script(type,hash).utxos() returns

1. An empty array if there are no utxos at the address
2. An array of one object with key utxos if there are are utxos at the address

So, typically, the dev is interested in response.utxos -- but this is undefined if there are no utxos (not an empty array).

Also include a comment about this in the function. Only way for new devs to learn this at the moment is run into the case and work through the errors.

let's pull <chronik url> out of the required command line params. not intuitive to always be customizing this.

default to an ABC server that can be adjusted by editing a constant in some file.

We'll be using this in many scripts, so it should be kept in its own file.

e.g. create a config.js and export this here