Add new stf doctor command to check if the versions of external dependencies are correct.

Add a `--devices` option to show a list of USB connected devices and a list of the devices that ADB can see.

lib/cli.js

@@ -1298,4 +1298,13 @@ program
       .done(run)
   })
 
+program
+  .command('doctor')
+  .description('diagnose issues before starting')
+  .option('--devices'
+    , 'diagnose devices connected to stf')
+  .action(function(options) {
+    require('./util/doctor').run(options)
+  })
+
 program.parse(process.argv)