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.
Add the theme 'conservation'.
Modify the line-height in the theme 'conservation'
Merge pull request #3 from xgfone/master

允许用户自定义配置文件