Add if option to conditionally hide a collection action

[guewen]

Here's a way to conditionally hide collection actions.
It may be useful to check permissions or even to hide actions depending on the current scope/filters as we have access to the current params[:q] and params[:scope].

Closes #41

[guewen]

Hi @workgena, thanks for this useful gem!

I proposed this PR and another one (#47), any chance to have them in a release once reviewed / tests ok?

I was not able to run tests locally (undefined method assets' for an instance of Rails::Application::Configurationwhen runningrake setup`), so I'm not sure tests actually pass.