Set scope highlight by ibl.setup_buffer

[ofseed]

Problem

rainbow-delimiters could be disabled for specific buffers, it would be better if indent-blankine's scope highlight is buffer-scoped so that we can only apply rainbow-delimiters' highlights to the buffers it enabled, and keep IblScope in the buffer rainbow-delimiters does not enabled.

Expected behavior

scope highlight is buffer-scoped and could be set by ibl.setup_buffer

[lukas-reineke]

While it would be possible with namespaced highlight groups, this would be quite complicated to implement, I think.

I'm not against merging a PR, but I don't think I will work on this myself.