Destination not used when installing nbextensions

[jhamrick]

The destination variable isn't used when installing nbextensions, even though the install_nbextension function supports it, and it's defined in the InstallNBExtensionApp class:

https://github.com/jupyter/notebook/blob/master/notebook/nbextensions.py#L703

It looks like it was removed in #879 here:

https://github.com/jupyter/notebook/pull/879/files#diff-720744abca8751e2ff12988ac308c604L308

[minrk]

Fixed by #1391