Allow to definte the element for the intro

[LukasK13]

#40

This allows to define the element on which the intro will be startet. In case of a modal dialog, this selector should be #shiny-modal.

[carlganz]

Thanks for the PR!

Initially my feeling was that any server side control of the intro should be manipulated by specifying the details of the tour in the options.

If I understand correctly the goal here is to be able to specify a static intro in UI and in modal, and then be able to programmatically from server select which static intro to initiate by specifying the the CSS selector of the first element. That seems totally reasonable, and I suspect the PR is fine as is.

I will need to look it over and test some edge cases, and I will likely tweak the javascript, but I will try to get this merged in when I find some time.

Thanks again!

[LukasK13]

Hi @carlganz

Sorry for my short explanations, but you understood my usecase correctly. If I wouldn‘t specify the element, the intro would appear behind the modal dialog.
As I‘m still a newbie at JS, I am pretty sure that there are lots of better solutions possible.

Thank you for your amazing work so far!