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.

g_docs.go

@@ -80,9 +80,7 @@ func init() {
 	importlist = make(map[string]string)
 	controllerList = make(map[string]map[string]*swagger.Item)
 	modelsList = make(map[string]map[string]swagger.Schema)
-	curPath, _ := os.Getwd()
 	astPkgs = map[string]*ast.Package{}
-	parsePackagesFromDir(curPath)
 }
 
 func parsePackagesFromDir(dirpath string) {