Added param option "show" to allow hiding parameters from documentation

### Example
```Ruby
api :POST "/cars", "add new car rating"
param :make, String, :desc => 'Make of the vehicle'
param :model, String, :desc => 'Model of the vehicle'
param :rating, [1,2,3,4,5], :desc => '5 star rating for the vehicle'
param :car_id, Integer, :desc => 'Legacy attribute still works but not shown in docs', :show => false
```
### Use cases
- Legacy parameter still supported but deprecated and you don't want new clients using it
- Advanced or internal use parameters that have very particular use cases that you don't want to document publicly for all clients to see
- Pilotting a new paramter/feature that is not ready for mainstream and you do not wish to show up in the documentation yet