Add optional functionality to pre process and extract parameters. 

### Description
The goal is to extract and pre process parameters of the request.

For example Rails, by default, transforms empty array to nil value, for some people it could be better to transform it again to an empty array. 

Or if we want to support a custom parameter type, like enumeration (comma separated values), this could be used to transform automatically the string parameter, representing an enumeration, to an array.

### Usage

To use it, set processing_value configuration variable to true.
```ruby
Apipie.configure do |config|
  config.process_values = true
end
```
In your controller action, use values variable instead of params.

To implement a custom pre processing, the idea is to write a `process_value` function in the validator:
function in your validator:
```ruby
   def process_value(value)
    value ? value.split(',') : []
 end
```