Feature: key or value prefix/postfix

[nicolkill]

Add to the schemas the posibility to add a prefix/postfix fixed to the keys or values

schema = %{
  field: %{
    "string:postfix:_id": :string,
    "string:prefix:order_:postfix:_id": :"string:prefix:external_value-"
  }
}

map = %{
  field: %{
    defined_postmortem_id: "any string value",
    order_defined_postmortem_id: "external_value-my_value_string"
  }
}