fix(slider): allow tabindex overwrite

[devversion]

This commit allows the user to specify the tabindex of the slider element.

If there are anymore suggestions or other wishes, please tell me 👍

Fixes #5829

[danjarvis]

Looks good. I would consider adding a small test for the fix:

it('should preserve tabindex', function() {
    var slider = setup('tabindex="2", ng-model="model"');
    expect(slider.attr('tabindex')).toBe(2);
}));

[devversion]

Done! Thanks @danjarvis for the suggestion.

[EladBezalel]

LGTM