Add flexibility to immediate destination format, of return file header.

Background: page OR146 of the NACHA 2013 Operating Rules and Guidelines discusses
return files, and says "See Appendix Three (ACH Record Format Specifications) for the File
Header" format. Within Appendix Three, page OR111 defines the immediate
destination format as bTTTTAAAAC, which is "a blank in the first
position, followed by the four digit Federal Reserve Routing Symbol, the
four digit ABA Institution Identifier, and the Check Digit."

However, the return files we receive do not adhere to this format, and
may contain other characters, as reflected in this commit.