enhancement: specify path into app-state when mounting with om/root

[mccraigmccraig]

when working with multiple om/root components it's common to want those components to receive a cursor anchored at different points in the app-state

this path could be specified as an arg to om/root

[sitepodmatt]

This would be very handy, a possible workaround is to encapsulate everything in one root and then use om/build to get the anchored cursors however this means surrendering possibly a larger area to Om then may be desired

[swannodette]

fixed, http://github.com/swannodette/om/commit/398429a5f5d0f3ae10e705c5ca0932592e1b994f