Option to 'fail' the coverage when coverage percentage is less than a specified minimum.

[drhuffman12]

Please:

• Add a command line option to bin/crystal-coverage for specifying a 'minimum coverage' value
• Change the bin/crystal-coverage script so that
  • It uses that command line option
  • After running the coverage script, it causes echo $? in the console to return a 1 (or higher) when the coverage percentage is less than that specified 'minimum coverage' value (so that it can optionally be used to fail builds that don't meet the specified minimum coverage).