Import custom meta for nav menu items.

[helgatheviking]

Support importing custom nav menu item meta by parsing the incoming meta into the core meta required (classes, description, etc) and everything else.

Closes #77.

To Test:

1. Add a custom field to Nav Menu items. Easiest approach might be to add a plugin such as my Nav Menu Roles.
2. Export all the site data (since exporting Menus alone is not possible)
3. Import to a new site. Should see the Nav Menu Roles settings persist in the newly created menu items.