Add nullable field types

[jdenen]

Current State

@ksherman doesn't like that he can't unset a field by explicitly setting it to null through APIs that define parameters with defparams. This happens because the library strips all null-value fields, regardless of how they're null.

With this PR

This PR introduces nullable field types (e.g. :string_nullable) that allow you to explicitly set a value to null and see that null value after validating params. Fields that are not explicitly set will continue to be ignored.

Example

defparams foo(%{
  one: :integer, 
  two: :integer_nullable
})

# explicit nil values for *_nullable type fields make it through
%{one: 1, two: nil} |> foo() |> Params.to_map()
#=> %{one: 1, two: nil}

# they don't make it through unless explicitly set to nil
%{one: 1} |> foo() |> Params.to_map()
#=> %{one: 1}

# non *_nullable type fields continue to ignore explicit nulls
%{one: nil, two: 2} |> foo() |> Params.to_map()
#=> %{two: 2}