feature: UISref should not clobber anchor tag's `onClick` handler

[christopherthielen]

If you write a UISref like this:

<UISref to="state">
  <a onClick={() => analytics('clicked!')>click me</a>
</UISref>

the UISref should call the anchor tag's onClick before executing its own logic.

Additionally, you should be able to halt a UISref by calling event.preventDefault() like this:

<UISref to="state">
  <a onClick={e => e.preventDefault())>nope, sorry</a>
</UISref>