Add support for highlighting variables

[Nek-12]

Many themes, including OneDark proposed in #33 contain highlighting of variables. We can find variables by assignment, val or other keyword preceding the declaration, or lack of braces after usage.

[tmaxxdd]

This can be easy to implement only when basing on these keywords for a given language. It would be great to have that so I will focus after Bugfixes. Thanks!