When no piece length is provided to imdl torrent create, a piece
length is selected based on the size of the input. The hueristic is
lifted directly from libtorrent.
Also adds a imdl torrent piece-length command, which prints a table
of the piece lengths chosen at different content sizes, which is useful
for understanding and debugging the piece length selection algorithm.
type: added
Colored output can be controlled on the command line with
`--use-color auto|always|never`. The default is `auto`, which enables
color if `imdl` detects that it is printing to a terminal.
Color can be disabled entirely by setting the `NO_COLOR` environment
variable.
type: added
