OpenURL doesn't respect base target markup

Hi

Trying to ensure that links webgl content in an iframe will always have the correct target but there is no target in the Unity script API.

Could you support the base target attribute so that this can be handled correctly by the container context?

http://stackoverflow.com/questions/...from-iframe-to-be-opened-in-the-parent-window

Adding target attribute to the OpenURL would also solve this.