Standardize metainfo property generation, setting, and skipping

[casey]

I'm noticing a common pattern with metainfo property flags to imdl torrent create. Many flags are concerned with setting a metainfo property (--source), avoiding setting a property, (--no-creation-date), or similar.

I'm considering changing argument handling to deal with these operations in a more uniform way:

• A --with flag, to include a property whose value has a reasonable default: --with creation-date
• A --without flag, to not include a property that would have been included by default: --without created-by
• A --set flag, to set a property to a particular value: --set source hello

Many of the flags to imdl torrent create could be removed in favor of these three flags. A huge bonus to this would be that these same flags could be reused for an eventual imdl torrent edit command, to add, remove, and set properties on existing torrents.