Allow setting content variables using a special syntax

By defining methods starting with `content_for_` and ending in `!`,
variables can be set at render time.

    class RocketPresenter < Curly::Presenter
      # Same as calling `content_for :rocket_name, "Apollo"` in an
      # ERB template.
      def content_for_rocket_name!
        "Apollo"
      end
    end