Allow administrate to be installed somewhere other than /admin

Created by: aaronrenner

I have a project that already has an /admin namespace and I'd like to install administrate as a secondary admin interface (maybe at /administrate).This would allow me to manage CRUD operations on models that don't have a UI yet. When I run rails g administrate:install it would be nice if there was an option to change the namespace of where I want it installed, so it doesn't conflict with my existing controllers. Here is an example of what rails admin does: https://github.com/sferik/rails_admin/blob/master/lib/generators/rails_admin/install_generator.rb#L13

I really appreciate you building this gem and I'm looking forward to using it.