One output flag for every command, with CSV alongside JSON

Every list and get command takes the same output flag now:

screenly --output json asset list
screenly --output csv screen list > screens.csv
screenly screen list                     # table, as before

--output (or -o) sits on the root command, so it works the same way everywhere rather than being a flag each subcommand had to declare for itself. That is what makes CSV possible without adding it fourteen more times.

CSV is the addition worth knowing about. A screen inventory that goes to somebody in facilities, or a monthly export into a spreadsheet, no longer needs a JSON parser in the middle of it.

The old --json flag still works and prints a deprecation warning. It goes to standard error, so a script redirecting output to a file gets clean data in the file and the warning on the terminal.