[FEATURE] Allow tip hiding on its outside click

This adds an option `hideOnTipOutsideClick` which will
hide the Tip if the document is clicked somewhere outside
the ToolTip resp. its initial trigger element.

Apparently this only makes sense if no contradicting
`hideOn` option is set, as it would be the case in most
`mouseleave` cases because there would be no opportunity
to click outside if the Tip is just hiding anyways.

Fixes #21
2.6 release
Fixed options.discrete scope
Fixed options.discrete scope
[TASK] Add composer.json manifest file
[BUGFIX] Removed incorrect z-index setting

see also https://github.com/lorenzos/FloatingTips/issues/24
[TASK] Allow not showing or hiding a tip automatically

This allows the `hideOn` and `showOn` options to also
be `null` if the tip should not be hidden or shown
on a particular event.

Besides, it corrects a variable scope.