Add a converter for Black Rank / White Rank

[artasparks]

We should add a converter for the BlackRank & WhiteRank properties + fields in the GameInfo struct in node.go: https://github.com/otrego/clamshell/blob/master/core/movetree/node.go

Extra care will need to be paid to ensure that comments are escaped correctly. In particular, ] characters should become \]. Currently the parser does the unescaping for comments.

• Property: https://www.red-bean.com/sgf/properties.html#BR, https://www.red-bean.com/sgf/properties.html#WR,
• Property example: BR[1k]
• Converters: https://github.com/otrego/clamshell/blob/master/core/prop/converter_list.go. The Move converter is probably a good one to look at.

Should have unit tests that test positive & error cases