config: add more method

DefaultString(key string, defaultval string) string      // support
section::key type in key string when using ini and json type;
Int,Int64,Bool,Float,DIY are same.
	DefaultStrings(key string, defaultval []string) []string //get string
slice
	DefaultInt(key string, defaultval int) int
	DefaultInt64(key string, defaultval int64) int64
	DefaultBool(key string, defaultval bool) bool
	DefaultFloat(key string, defaultval float64) float64
	DIY(key string) (interface{}, error)
	GetSection(section string) (map[string]string, error)
	SaveConfigFile(filename string) error