Positional arguments should be replaced by named options

[TimDaub]

• Right now, we're using positional arguments: cargo run -- submit-ledger https://warpcast.com/timdaub/0x1f54f8 "Kiwistand is live", where the first argument is "submit-ledger" (which is OK), but then to a user it can be confusing that they first have to add the link, and second the title [link] [title]
• Rather, I'm used to submit named arguments, e.g. cargo run -- submit-ledger --url=https://... --title="Kiwistand is live", this makes command construction less error prone and more readable