Issue title: slider's z-index
 Issue body: It appears on top of everything in the page even when given a negative z-index, how to fix this? I have a pop-up box opening on my site and I wish to disply text in the box but the slider is in the way.
How can I fix this?
 Comments: 
 Comment 0: Have you checked the CSS definitions in the source? There you can see that `.noUi-base` and `.noUi-handle` have a default z-index of 1. So just set a higher z-index for your pop-up box and it should appear on top of the slider. I tested it and it works for me.
 Comment 1: Yea, I changed those to -1 and my pop-up to 1, it's still above the pop-up and im confused.
check out my site www.ubit.ca, when you click the 'View Keys' button.
the style.css and nouislider.css are in the css folder www.ubit.ca/css
 Comment 2: I meant add a higher z-index for your pop-up box INSTEAD of setting a negative z-index for the slider. Everything seems fine if you do it my way, see [screenshot](https://imgur.com/a/Q6Eao). The slider works if you disable the negative z-index and the popup is on top of the slider (I just moved it with devtools).
 Comment 3: The pop-up i'm talking about is the one that you open when you click "view keys", im trying right now, let me see.
 Comment 4: This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.
{'source': 'GithubIssues', 'repo_name_with_owner': 'leongersen/noUiSlider', 'issue_id': 260485093, 'repo_stars': 5729.0}