Add support for colorized parentheses

[su8]

When using nested parentheses it would be a good idea to use https://github.com/luochen1990/rainbow

[JBakamovic]

Any plugin can be integrated simply by cloning it into the <yavide_installation_dir>/core/external directory. Plugin-related configuration shall be then put into <yavide_installation_dir>/config/common.plugin. Reading the rainbow documentation one can see that we need to let g:rainbow_active = 1 in order to get it running. I've tried it right now and it works.

What I personally think is that it would be interesting to get this feature running natively within the semantic-syntax-highlighting service. It fits into the use-case (purpose of the service existing) perfectly.