(v4) unable to override popover text color

[changLiuUNSW]

I change $popover-bg to black and also want to change text color to white, but I cannot find variable for popover text color.

I suggest we should expose a variable for popover text like $popover-color

[praveenpuglia]

I would suggest adding two variables with the following defaults.

$popover-title-color   : $headings-color !default;
$popover-content-color : $body-color !default;

I can send a PR if adding it is finalized.

[praveenpuglia]

I sent the PR anyway. If the ability to change color doesn't need to be present, please ignore it.