Allow redefining to_param delimiter using param_delimiter

[nvasilevski]

This commit allows customizing the delimiter used by to_param when to_key returns multiple value. This unblocks supporting more varieties of composite primary key types in Active Record.

This PR has a two commits:

1. Active Model addition - adds param_delimiter attribute to configure the delimiter in Active Model
2. Active Record change - tweaks to_param to support composite primary key and utilizes newly added param_delimiter along with defining AR default delimiter to _

It is possible to add composite primary key support to to_param without the delimiter abstraction by hardcoding certain value but it's really hard to choose one since we can't account for all possible values of the composite primary key values to avoid conflicts. And asking applications to override whole to_param seems inconvenient

I would be more than glad to split these changes into two separate PR and avoid squashing as I would definitely like to keep these commits separated