Allow customization of has_many link parameters

I have a case where I need the parameters for has_many links to be different than the default. 

Given the models Project and User where a user has many projects, rest pack would include a link with an href like `/projects?user_id={users.id}`. I'd like to do something like `/projects?owner={user.name}`. 

This PR just allows a hash of options to be included with each link passed  to `can_include` like so:

```ruby
class UserSerializer
  include RestPack::Serializer

  can_include projects: { param: "owner", value: "name" }
```