Add extra_options from CLI

src/kemal/cli.cr

@@ -34,6 +34,7 @@ module Kemal
           puts opts
           exit 0
         end
+        @config.extra_options.try &.call(opts)
       end
     end
 

src/kemal/config.cr

@@ -7,7 +7,7 @@ module Kemal
     @server : HTTP::Server?
 
     property host_binding, ssl, port, env, public_folder, logging,
-      always_rescue, serve_static, server
+      always_rescue, serve_static, server, extra_options
 
     def initialize
       @host_binding = "0.0.0.0"
@@ -68,6 +68,9 @@ module Kemal
       HANDLERS.insert(0, @logger.not_nil!)
     end
 
+    def extra_options(&@extra_options : OptionParser ->)
+    end
+
     private def setup_error_handler
       if @always_rescue
         @error_handler ||= Kemal::CommonExceptionHandler.new