Support serial number format params

They can be the substitute for format param, and promote few more
maintenanceability than too long format param.

I want to parse the rails log as below.

```
Started GET "/users/" for 127.0.0.1 at 2013-06-14 12:00:04 +0900
Processing by UsersController#index as HTML
  Rendered users/index.html.erb within layouts/application (0.5ms)
Completed 200 OK in 821ms (Views: 819.5ms | ActiveRecord: 0.0ms)


Started GET "/users/123/" for 127.0.0.1 at 2013-06-14 12:00:11 +0900
Processing by UsersController#show as HTML
  Parameters: {"user_id"=>"123"}
  Rendered users/show.html.erb within layouts/application (0.3ms)
Completed 200 OK in 4ms (Views: 3.2ms | ActiveRecord: 0.0ms)
```

I wrote a such Regexp.

```
format /Started (?<method>[^ ]+) "(?<path>[^"]+)" for (?<host>[^ ]+) at (?<time>[^ ]+ [^ ]+ [^ ]+)\nProcessing by (?<controller>[^\u0023]+)\u0023(?<controller_method>[^ ]+) as (?<format>[^ ]+?)\n(  Parameters: (?<parameters>[^ ]+)\n)?  Rendered (?<template>[^ ]+) within (?<layout>.+) \([\d\.]+ms\)\nCompleted (?<code>[^ ]+) [^ ]+ in (?<runtime>[\d\.]+)ms \(Views: (?<view_runtime>[\d\.]+)ms \| ActiveRecord: (?<ar_runtime>[\d\.]+)ms\)/
```

It's too long, and maintenance of this Regexp seems to ugly. This modification enables to write the same Regexp as below.

```
format1 /Started (?<method>[^ ]+) "(?<path>[^"]+)" for (?<host>[^ ]+) at (?<time>[^ ]+ [^ ]+ [^ ]+)\n/
format2 /Processing by (?<controller>[^\u0023]+)\u0023(?<controller_method>[^ ]+) as (?<format>[^ ]+?)\n/
format3 /(  Parameters: (?<parameters>[^ ]+)\n)?/
format4 /  Rendered (?<template>[^ ]+) within (?<layout>.+) \([\d\.]+ms\)\n/
format5 /Completed (?<code>[^ ]+) [^ ]+ in (?<runtime>[\d\.]+)ms \(Views: (?<view_runtime>[\d\.]+)ms \| ActiveRecord: (?<ar_runtime>[\d\.]+)ms\)/
```