Add command to initiate garbage collection

shell/shell.gyp

 {
   'targets': [{
     'target_name': 'abpshell',
     'type': 'executable',
     'dependencies': [
       'libadblockplus.gyp:libadblockplus'
     ],
     'sources': [
       'src/Main.cpp',
       'src/Command.cpp',
+      'src/GcCommand.cpp',
       'src/HelpCommand.cpp',
       'src/MatchesCommand.cpp',
       'src/SubscriptionsCommand.cpp'
     ]
   }]
 }