Allow the user to assign the config file.

First, if the command line offers the option, '--config-file PATH',
assign 'PATH' to beego.AppConfigPath; or, use the environment
'BEEGO_APP_CONFIG_FILE'. If neither exists, use beego's default value.

In the config file, the user maybe define 'ViewsPath' and 'StaticDir'.
Default, they use the 'views' and 'static' in the current directory.