Allow using a custom page param with pagination

[paulcsmith]

Do this after #1020

  # The name of the param that Lucky will use for the current page. Defaults to `:page`.
  #
  # You can override this method to use a different page param. This is rarely
  # overridden, but you can if you need to.
  #
  # ## Example
  #
  # ```crystal
  # abstract class BrowserAction < Lucky::Action
  #   include Lucky::Paginator::BackendHelpers
  #
  #   def paginator_page_param : Symbol
  #     :my_custom_page_param
  #   end
  # end
  # ```
  def paginator_page_param : Symbol
    :page
  end

And also add an arg to Paginator so it can use the param when generating links!

[wout]

Could be very useful. Sometimes you might want to use :p instead for :page, just like using q instead of query.