Rename "format" to "output format"

[chris48s]

I'd like to bring this into line with the terminology in the plugin system.

I think the way I will do this is:

• In a minor release, add an --output-format command line flag and an outputFormat key in the config file
• Make them do the same thing as --format and format
• Mark --format and format deprecated in the changelog
• Mark format deprecated in the JSON schema
• Mark --format as deprecated using yargs
• Log a deprecation warning to the console if either of those things are used

Then in the next major, actually remove format and --format