Added property for pin label.

[Inrego]

Solves #133 although with a little more customization than suggested there.

I've added an optional pinLabel property that if set, will be used for the pin label instead of just the immediateValue. That way, you can create your own text with bindings, like so:

immediate-value="{{durationSliderValue}}" pin-label="[[_readableDuration(durationSliderValue)]]"

If not set, the value will be used as normal, so no breaking changes.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please let us know the company's name.

[Inrego]

I've signed the CLA.

[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for the commit author(s). If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.