feat: add borderClass prop to allow a custom border class

[danny-does-stuff]

Allow overriding the default border class name "border" with a custom class name. Currently, when using a border it adds the class name "border", which is pretty generic and can be overridden by other styles (notably, bootstrap uses the class "border", overriding any borderColor provided to react-tooltip). This change makes it so that the user can provide a custom class name for the border class.