Export to plain text should allow attributes

[leeper]

Currently export of a rich binary format (e.g., .sav or .dta) to a plain-text format (e.g., .csv) drops all data frame-level and variable-level metadata. Using .csvy will store this in a header, but presumably most people aren't using that. It would be nice to be able to store value labels (as in exporting a factor) not just the numeric codes.