Add a converter for Comment property

[artasparks]

We should add a converter for the Comment property + a field in the node 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#C
• Property example: C[blah]
• 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