add "ruby" keyword 

This pull-request is for #195

* to pass Kernel functions
```ruby
file_path_list = ruby.open("/path/to/log_file.list"){|f| f.readlines}.map(&:chomp)
file_path_list.each do |path_string|
  source {
    type "tail"
    path path_string
    ...
  }
end
```

* to provide blocks with normal ruby syntax/features
```ruby
file_path_list = ruby {
  require 'net/http'
  res = Net::HTTP.start("ourserver.local", 80) {|http|
    http.get('/server_list.txt')
  }
  res.split(/\r?\n/)
}
file_path_list.each do |path_string|
  ...
end
```