[MIXLIB-30] Malachheb 1.4.0

I added the argument 'in' to specify the valid values ​​of an option like this : 

    option :log_level,
      :description => "Set the log level (debug, info, warn, error, fatal)",
      :in => ['debug', 'info', 'warn', 'error', 'fatal']

I think this feature is important for some options,
Thanks.
would be good to do some formatting of ` #{opt_value[:in]}`, for example:

```ruby
" '#{opt_value[:in].join("', '")}'"
```

To make it more clear what the valid options are. Otherwise, you get the default `Array#to_s` behavior which is:

```ruby
%w{foo bar}.to_s
# => "[\"foo\", \"bar\"]"
```