Add possibility to define reusable params.

Another one concept :)

Examples:
```ruby
helpers do
  params :pagination do
    optional :page, type: Integer
    optional :per_page, type: Integer   
  end
end

desc "Get collection"
params do
  use :pagination
end
```
using module:
```ruby
# shared_params.rb
module SharedParams
  extend Grape::ApiHelpers

  params :period do
    optional :start_date
    optional :end_date
  end

  params :pagination do
    optional :page, type: Integer
    optional :per_page, type: Integer   
  end
end
```

```ruby
helpers SharedParams

desc "Get collection"
params do
  use :pagination, :period
end
```