Add --output_fp to print_metadata_stats.py

Add a new option to save the results of the script in a file. Add a new
script usage example with a corresponding update to the test_data
directory.

This is in favor the integration with Galaxy and should fix item 3 of #633
Merge pull request #871 from ElDeveloper/print_metadata_stats_633

Add --output_fp to print_metadata_stats.py