Print out usage information

Adding a `--help` option which prints out the flags and their default values as well as a short example.

    $ greyhound --help
    greyhound: Fast fuzzy filepath finder.
    
      -dir="": Directory to search in.
      -exclude="": Sub directories to exclude from search.
      -help=false: Shows this message.
      -query="": File path pattern to search for.
    
     e.g. greyhound --query test --exclude .git,.svn --dir ~/code/project