add explicit encoding mode to file_converter/txt.py

[Zenahr]

Related to #477, #223.

This would just be a quick fix. The default encoding mode is set to utf-8. Users would still have to change the source code in order to switch to other encoding modes.

[tholor]

Hi @Zenahr ,
Thanks for adding this! Encoding issues are always nasty. How about we add a parameter encoding to the __init__ so that users can pass the required encoding if needed? We could still put utf-8 as the default there.

[Zenahr]

@tholor sounds sensible to me. Would you mind adding it yourself? I can't access my development machine atm. Or I'll add it later myself.

[tholor]

Sure, I'll add it :)

[tholor]

After a second thought, I decided to add the param rather to the convert() method, as the encoding might change for different files. Therefore, a class attribute seemed too static.