Use "brew deps --installed" to get dependencies of installed formulae
Use Hash instead of OpenStruct
Parse command line arguments during initialization
Provide --all and --installed options

Replace existing --graph option with --all and --installed switches, similar to the "brew deps" command
Get rid of raise

OptionsParser already makes sure that only valid formats are accepted.
Fix indentation
Implement to_s in Dot and GraphML
Add OptionParser banner
Allow additional formula argument

Similar to the "brew deps" command, if an additional argument is specified, it is treated as the name of a formula for which the dependency graph is created. The --installed and --all switches take precedence, however
Rename some method arguments
Find more meaningful name for method