Add 'is.*' command group

[pyroscope]

See also https://github.com/rakshasa/rtorrent/issues/591

• Operator group like elapsed.* or math.*
• At minimum is.eq, is.lt, is.gt, is.ne, is.le, is.ge
• Maybe is.like for matches (and use SQL syntax 'cause like?!)
• Accept literal values as arguments, either value or string
• Accept 1-2 args: 1 = cmp TO against literal, 2 = compare literals
• If one arg and it is a list, then compare two torrent objects using the given field names in order
• Cast literal to object type, then compare