allow (but ignore) a leading "[no-]" before flag name in @synopsis

`no-` prefix on flag name sets it to `false`, but there was no valid way to show this in usage synopsis. Showing this is relevant for flags with a dynamic default value (like `--color` config flag).  This change makes `@synopsis [--[no-]flag]` valid.
Merge pull request #754 from smhmic/patch-2

allow (but ignore) a leading "[no-]" before flag name in @synopsis