* escape special chars in csv and json output.

- escape \b,\f,\n,\r,\t,\," from strings before dumping
  them to json or csv.
- also faithfully reproduce the sign of nan in json.
this fixes github issue #745.

* functionalize.

* split string escape functions between csv and json

* Update src/csv_reporter.cc
Co-Authored-By: tesch1 <tesch1@gmail.com>

* Update src/json_reporter.cc
Co-Authored-By: tesch1 <tesch1@gmail.com>