Custom decoder

[davidnewhall]

• Closes Filenames appear as garbled text when extracting. #72

quick example:

x := &xtractr.XFile{
	FilePath: "/tmp/file.zip",
	Encoder: func(input *xtractr.EncoderInput) *encoding.Decoder {
		return encoding.Nop.NewDecoder()
	},
}

One day I'll write a character encoding detector that actually works.