Add charset detection to decoder

[i39]

Add charset detection for remove errors like this:
extractor.go:58: xml: encoding "windows-1251" declared but Decoder.CharsetReader is nil

[gnewton]

Hello,

Sorry to be so slow to respond.

I checked out https://github.com/i39/chidley and when I did 'go build' got these errors:

$ go build
# _/home/gnewton/tmp/g/chidley
./extractor.go:42: undefined: charset.NewReaderLabel
$

I will look at your suggested solution and see if I can implement it.

Thanks,
Glen