support sort function

[qianxiansheng90]

Option struct like this:

// Options is Pretty options
type Options struct {
	// Width is an max column width for single line arrays
	// Default is 80
	Width int
	// Prefix is a prefix for all lines
	// Default is an empty string
	Prefix string
	// Indent is the nested indentation
	// Default is two spaces
	Indent string
	// SortKeys will sort the keys alphabetically
	// Default is false
	SortKeys bool
	// sort function
	SortFunc func(level int,i, j []byte) bool
}

The output format can be customized using the sort function.