Add flexible encoding support for the result

[wqh17101]

@mchehab Right now. ZBar use hardcode to set the encoding supported and priority of the coding of the result. As in the https://github.com/mchehab/zbar/blob/89e7900d85dd54ef351a7ed582aec6a5a5d7fa37/zbar/qrcode/qrdectxt.c#L83-L89 set the encoding zbar support

and as in the https://github.com/mchehab/zbar/blob/89e7900d85dd54ef351a7ed582aec6a5a5d7fa37/zbar/qrcode/qrdectxt.c#L186-L189 set the encoding priority

[mchehab]

If I understood well, in order to address this issue, ZBar needs to:

1. honor LC_ALL and LC_LANG environment vars, using it by default;
2. add more encodings, for Asian charsets, as, currently, only Japanese Big 5 is supported.
3. provide a way for he user to explicitly set the charset.

for (3), probably a new API function will be needed, in order for the ZBar executables to pass such options to ZBar core.