Support additional options as part of the Config

This commit adds the ability to add a closure suitable for adding
additional options. It is expected to allow someone to set global,
module or class level variables so they can pass changes/options
suitable for making decisions.

src/kemal/cli.cr

@@ -35,6 +35,10 @@ module Kemal
           puts opts
           exit 0
         end
+        unless @config.on_options.nil?
+          c = @config.on_options.not_nil!
+          c.call opts
+        end
       end
     end
 

src/kemal/config.cr

@@ -3,7 +3,7 @@ module Kemal
     INSTANCE = Config.new
     HANDLERS = [] of HTTP::Handler
     property host_binding, ssl, port, env, public_folder, logging,
-      always_rescue, error_handler, serve_static, run
+      always_rescue, error_handler, serve_static, run, on_options
 
     def initialize
       @host_binding = "0.0.0.0"
@@ -16,6 +16,7 @@ module Kemal
       @always_rescue = true
       @error_handler = nil
       @run = false
+      @on_options = nil
     end
 
     def logger