Add an operator to export all nested fields for CSV output format

mongoexport currently projects the entire top-level portion of a document for fields containing a ".". As the entire top-level is projected for dot-delimited field names it would be useful to add an operator like a.$$$.b to iterate on a and extract all values for the nested b fields and write additional lines to output with the CSV representation. This should also be doable in the case if a is an array.