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

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.

Start by tracing how the existing `hl` and `selected-hl` highlight groups are defined and applied in fzf. Add support for configurable `hl-bg` and `selected-hl-bg` groups, then verify that highlighted substrings can use background colours independently from foreground colours.

Written by the indexing model from the issue text.