Modified padrino start to take an extra --options (-O) parameter

to pass options to the rack handler.

For instance, with `puma` you can now do:
`bundle exec padrino s -O Threads=3:16` and the server will start with 3
to 16 threads.

This implementation follows the same syntax that
[rackup](https://github.com/rack/rack/blob/master/lib/rack/server.rb\#L100) uses for
compatibility.

Also implemented --server_options to print out the supported options for
the selected handler (application server).

This has been requested a couple of times, the latest in #1451.

I'd still recommend people to run their applications through the
application server directly -e.g., puma, unicorn, thin...- instead of
using this but it might come in handy for development.

padrino-core/lib/padrino-core/cli/base.rb

@@ -18,11 +18,19 @@ class Base < Thor
       method_option :daemonize, :type => :boolean, :aliases => "-d", :desc => "Run daemonized in the background."
       method_option :pid,       :type => :string,  :aliases => "-i", :desc => "File to store pid."
       method_option :debug,     :type => :boolean,                   :desc => "Set debugging flags."
+      method_option :options,   :type => :array,  :aliases => "-O", :desc => "--options NAME=VALUE NAME2=VALUE2'. pass VALUE to the server as option NAME. If no VALUE, sets it to true. Run '#{$0} --server_options"
+      method_option :server_options,   :type => :boolean, :desc => "Tells the current server handler's options that can be used with --options"
+
       def start
         prepare :start
         require File.expand_path("../adapter", __FILE__)
         require File.expand_path('config/boot.rb')
-        Padrino::Cli::Adapter.start(options)
+
+        if options[:server_options]
+          puts server_options(options)
+        else
+          Padrino::Cli::Adapter.start(options)
+        end
       end
 
       desc "stop", "Stops the Padrino application (alternatively use 'st')."
@@ -124,6 +132,29 @@ def prepare(task)
         end
       end
 
+      # https://github.com/rack/rack/blob/master/lib/rack/server.rb\#L100
+      def server_options(options)
+        begin
+          info = []
+          server = Rack::Handler.get(options[:server]) || Rack::Handler.default(options)
+          if server && server.respond_to?(:valid_options)
+            info << ""
+            info << "Server-specific options for #{server.name}:"
+
+            has_options = false
+            server.valid_options.each do |name, description|
+              next if name.to_s.match(/^(Host|Port)[^a-zA-Z]/) # ignore handler's host and port options, we do our own.
+              info << "  -O %-21s %s" % [name, description]
+              has_options = true
+            end
+            return "" if !has_options
+          end
+          info.join("\n")
+        rescue NameError
+          return "Warning: Could not find handler specified (#{options[:server] || 'default'}) to determine handler-specific options"
+        end
+      end
+
       protected
 
       def self.banner(task=nil, *args)

padrino-core/lib/padrino-core/server.rb

@@ -31,6 +31,7 @@ def self.start(app, opts={})
         FileUtils.mkdir_p(File.dirname(options[:pid]))
       end
       options[:server] = detect_rack_handler if options[:server].blank?
+      options.merge!(Hash[*options.delete(:options).map {|opt| opt.split('=',2)}.flatten].symbolize_keys!) if options[:options].is_a?(Array)
       new(options, app).start
     end