Add `-o` flag for `export` command (#1035)

* Add `-o` flag for `export` command

The export command now prints to standard output by default.  To store the
output in a file, the new `-o <filename>` argument can be used.

* Fix code style

* Improve usage examples for `export` command

* Add missing import statement to exporter module

* Make Exporter class work w/ IO objects for output

* Fix test suite for `Exporter` utility class