Allow specifying background colour of the `hl` and `selected-hl`

[savchenko]

It doesn't seem to be possible to specify background of the highlighted substrings.

This prevents from highlighting content via the change of BG colour instead of the FG.

For example, this isn't doable:

Please consider adding hl-bg and selected-hl-bg highlight groups.

[junegunn]

For the moment, you can use reverse attribute to achieve a similar effect.

fzf --color fg:black,bg:white,hl:yellow:reverse,hl+:bright-cyan:reverse

[savchenko]

Does not seem to work as intended?