Enhancement: Allow developer to set FlashName and FlashSeperator values

Hello @Astaxie,

When building an application, there was a requirement for me not to leak what type of framework was being used. I made a patch that allows the user to set the Flash values in the .conf file or the main.go, so as not to indicate that beego was being used.

This is just a first purposal to see if you are open to the change. If I need to modify something let me know.

```go
func main() {
  beego.Router("/", &controllers.MainController{})
  beego.FlashName = "CUSTOM_FLASH_NAME"
  beego.FlashSeperator = "CUSTOM_FLASH_SEPERATOR"
  beego.Run()
}
```
Thanks,
Jared