Add new feature to persist the value after a refresh

[jespirit]

Closes #901.

The user can pass an object with the property useCurrentValue set to true so that the slider maintains its value after a call to refresh.

I will add an example as well. Done.

Pull Requests

Please accompany all pull requests with the following (where appropriate):

• [x] 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
• [x] Link to original Github issue (if this is a bug-fix)
• [x] documentation updates to README file
• [x] 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