refactor(popover): allow string selector as `positionTarget`
refactor(popover): allow string selector as `positionTarget`

jnizet/ng-bootstrap,ng-bootstrap/ng-bootstrap,ng-bootstrap/ng-bootstrap,fbasso/ng-bootstrap,fbasso/ng-bootstrap,fbasso/ng-bootstrap,jnizet/ng-bootstrap,jnizet/ng-bootstrap,ng-bootstrap/ng-bootstrap,jnizet/ng-bootstrap,ng-bootstrap/ng-bootstrap,fbasso/ng-bootstrap