Recognise variable references in *.vim themes

[savchenko]

Hopefully this is "easier" to implement for .vim rather than for other formats. On the screenshot below hex colours are rendered as expected, however the g:terminal_color_background isn't highlighted:

This would be quite useful when editing theme files so that one can see colours "live" instead of bare variables:

[chrisbra]

I am sorry, this requires semantic understanding of Vim script. That is far out of scope for a simple colorizer I fear.