Support cancel extension within \ce

[polyluxus]

It would be great to have the cancel extension somehow supported. Here is a real world example on chemistry.se: Why did they ‘split’ this H2+ ion in the ionic equation?

The current work-around for me was to nest the commands:

\ce{Mg(s) + H2+(aq) + \cancel{\ce{SO4^{2-} (aq)}} 
    &-> Mg^{2+}(aq) + \cancel{\ce{SO4^{2-} (aq)}} + H2(g)}