'winhl' is being overwritten

[romgrk]

In my config files, I set the winhl option for LuaTree when it starts, but it's being overwritten. LuaTree should use set winhl+= to preserver existing options.

Btw, as soon as you merge the refactor PR I plan to help.

Below is a screenshot of the reason why I wanna modify it. Had to monkey-patch some files to make it.

[kyazdani42]

I agree some config choices are not as good as they could be, i'll remove the override in the config too (or at least provide a proper Hl groups to highlight those, which is not obvious right now)