Validate string in list

[cnavas88]

• Adding "included" in String validator.
• Validate that a string is included in a list.
• Testing Included function.
• Documentation included function.

[dmlittle]

@cnavas88 thanks for the PR! I have some comments on the implementation and naming convention of this feature but this is something we'd love to support! @mgartner might have more thoughts on this issue.

[dmlittle]

I like this feature but I think I'd prefer having the name option be a list called :valid since it specifies the list of valid strings. This would make the using this feature like this:

schema = %{
  "casing" => %Litmus.Type.String{
    valid: ["lowercase", "uppercase"]
  }
}

[mgartner]

@dmlittle i think you tpyo'd included instead of valid 😆

[dmlittle]

I think it might be helpful to return the valid values in the error to provide a more helpful message. Taking inspiration from Joi we could do something like this:

defp included(%__MODULE__{valid: valid}, field, params) do
  if Map.get(params, field) in valid do
    {:ok, params}
  else
    {:error, "#{field} must be one of #{valid}"}
  end
end