Add a simple Crawler example

[Quantumplation]

While working on Amaru, we'll likely want to grab lots of example data. This adds a small utility to easily grab one block, one tx, or blocks/txs matching some predicate.

Example usage:

• implement tx_matches or block_matches at the top of the file
• run on a machine with a local cardano node
• For command line arguments, see:

cargo run --bin crawler -- --help