Add coverAxes option to crosshair plugin.

If you have multiple axes, it is useful to let the crosshair also show over the axes space. In this implementation, that means the whole canvas width or height. You can enable it either in the "x" or "y" direction or both, the same way that "mode" works. 
![coverAxes](https://f.cloud.github.com/assets/1271422/270652/bf8985d8-8fd0-11e2-8642-e53d9b7c5dc9.png)