Adds the capability to set certain attributes as an item is being added.
For instance to set the label at the same time as adding a torrent:
Added the types Field and Field Value.
- Field represents the current supported (and used) attributes by this library on "Download Items" (d.* commands) and "File Items" (f.* commands)
- FieldValue is a convenience type to hold and convert a particular Field that one wants to set on a rTorrent item.
Thanks to @Burmudar for this contribution.