Change operator color to brown in gruvbox theme

[rsjethani]

This would help differentiating operators from constants and builtins.
We do this by adding new brown color to the palette and using it as
foreground color for operators.

Go based example:

Before:

After:

[the-mikedavis]

This gruvbox tries to follow the upstream gruvbox as closely as possible: https://github.com/morhetz/gruvbox

The new brown isn't in the palette as far as I can see https://github.com/morhetz/gruvbox/blob/bf2885a95efdad7bd5e4794dd0213917770d79b7/colors/gruvbox.vim#L89-L132. It looks like upstream highlights operators as fg1: https://github.com/morhetz/gruvbox/blob/bf2885a95efdad7bd5e4794dd0213917770d79b7/colors/gruvbox.vim#L590-L591, https://github.com/morhetz/gruvbox/blob/bf2885a95efdad7bd5e4794dd0213917770d79b7/colors/gruvbox.vim#L465.