Added PreRun phase to Command struct

Now each command has a PreRun function that will execute before calling
the Run() function. This allows to show the banner and do some pre-check
work. Also moved parsePackagesFromDir() to the main function to avoid
getting called each time 'bee' is invoked.

hproseapp.go

@@ -298,14 +298,13 @@ var hproseAddFunctions = []string{}
 
 func init() {
 	cmdHproseapp.Run = createhprose
+	cmdHproseapp.PreRun = func(cmd *Command, args []string) { ShowShortVersionBanner() }
 	cmdHproseapp.Flag.Var(&tables, "tables", "specify tables to generate model")
 	cmdHproseapp.Flag.Var(&driver, "driver", "database driver: mysql, postgresql, etc.")
 	cmdHproseapp.Flag.Var(&conn, "conn", "connection string used by the driver to connect to a database instance")
 }
 
 func createhprose(cmd *Command, args []string) int {
-	ShowShortVersionBanner()
-
 	w := NewColorWriter(os.Stdout)
 
 	curpath, _ := os.Getwd()