Highlighting broken with non-standard bright terminal colors

**Describe the bug**

#1656 made the selected line "bright". But it is not safe to assume that the "bright" colors are actually brighter versions of the base colors. This is not an actual standard and some color schemes may use the "bright colors" for entirely different colors, which makes the hightlighting in lazygit broken.

**Expected behavior**

An option to make the selected line *not* bright (off by default) would solve the issue.