Adds the command `imdl` torrent verify` to verify the contents of torrents.
This implementation is extremely naive. It does successfully verify torrents,
but it will produce unsatisfying results when a torrent fails verification. In
particular, it won't give any information about which pieces in a file were
corrupt.
type: added
Support adding a `source` key under the `info` dictionary. The `source`
key is commonly used to create distinct torrents for different trackers.
type: added
The `imdl torrent show` command displays information about on-disk
torrent files. The formatting of the command's output is copied from
torf, an excellent command-line torrent creator, editor, and viewer.
type: added
