Allow to create GS1 DataMatrix

[awiz911]

At the moment it doesn't allow to do that. Because it doesn't understand FNC1 at the start of string and you can't pass it, it will split it in 2 bytes at best.

However this peace of code will solve it easily(checked and doublechecked):

instead of (in datamatrix package in encoder.go)

func encodeText(content string) []byte { var result []byte

it's possible to init result like so:

result := []byte{232}

this way it doesn't get processed with other content and we have required FNC1 at the start of our DataMatrix
sending FNC1 in initial content is problematic, cuz it reads it as 2 bytes and gives double FNC1 in result at best

p>s> guess it would be best to do this in separate package DataMatrixGS1 to keep backward compatability ))) cuz some scanners might not recognize it