This PR fixes #649 and in some sense fixes #952.

I initially tried the second or third approach outlined in #952, but realized that it would be difficult to actually use Popper.js. createPopper requires a DOM element to anchor the tooltip to, and in some uses of bootstrap-slider, there isn't a tick associated with the position being tooltipped. We could create an artificial tooltip position DOM element to give to createPopper, but I instead tried a fourth approach:

4. Implement the logic of preventOverflow manually.

This achieves the goal of no dependencies, adding a fairly small amount of code for nice functionality, hidden within a new preventOverflow option. I didn't change the defaults because it would break a lot of tests and backward compatibility, but happy to change.

I did not implement flip yet, mainly because I care about it less. I could work on it if desired.

I've gotten this working well in my project (via npm link), which uses left-to-right horizontal sliders and always-shown tooltips. Here are some screenshots

context	screenshot
left extreme
right extreme (including scrollbar)
in the middle

I should mention that the logic I implemented is much simpler than Popper's preventOverflow.js which seems to be mainly dealing with the case of clipping to the nearest display: fixed ancestor. I opted to just clip to window. There may be some subtleties I'm missing.

As this is my first contribution, I could use some guidance on how to test more thoroughly/broadly, and these PR requests:

• unit tests (we use Jasmine 2.x.x)
• JSFiddle (or an equivalent such as CodePen, Plunker, etc) or screenshot/GIF with new feature or bug-fix
• Link to original Github issue (if this is a bug-fix)
• documentation updates to README file
• examples within /tpl/index.tpl (for new options being added)
• Passes CI-server checks (runs automated tests, JS source-code linting, etc..). To run these on your local machine, type grunt test in your Terminal within the bootstrap-slider repository directory