Support mktoString rendering in text versions of emails

Straightforward request: because mktoString variables can be referenced inside editable mktoText areas in emails, they automatically are copied into the text version of emails. However, because the text version of the email does not process email module variables, only the variable name itself shows up. Can the text version please copy the variable when the variable is a string? It's understandable that others don't copy over, but strings are valuable in a variety of contexts.