Treat all ZIP codes as text strings in CSV export

When creating a CSV export from a contact search, (US) zip codes (and +4) beginning with a 0 are not enclosed in "". As a result, when they are imported in Excel or Google sheets, they are not treated as text, imported as numbers, and the leading 0 is lost. Excel does have the ability to define the format of each imported column IF the File->Import feature is used, but on a Mac, in the latest Excel, you can't scroll to the right when setting the format for each column, meaning that if the zip code column is column #10, you can't select it to set its format. That is a bug in Excel, but I do think we should fix this in CiviCRM as well, so that imports work "out of the box" and don't require file->import to import zip codes correctly.