Allow pondering variations to be sent to Sabaki

[ghost]

Some enhancements for playing a game against leelazero and reviewing straight after. @yishn

Gives more options:

• '--black' and/or '--white' to send that color variations to Sabaki. Ponder variations for 'play' as well as 'genmove' now can get sent to Sabaki.

• '--limitdepth' truncates the variation length, it is easier to see the main moves especially with '--flat'. It would be nice to specify 'depth n', but that would need to change how command line args work when calling leela. mybe put leela and leela_args in a < ... >, but that might be confusing to the user.

• '--labels' sends the first move of each variation as a label A, B, C, ... for Sabaki to update the root gametree. useful with the '--flat' option to easily see where the sibling variations are.

[yishn]

How about having the syntax --limit={n}?