Fix attribute processing in the LiblouisBrailleFilter

[joeha480]

Attribute processing in the filter(Translatable x) method could be improved to include attributes not provided by Liblouis itself, such as sub, sub and dd.

https://github.com/brailleapps/dotify.translator.impl/blob/master/src/org/daisy/dotify/translator/impl/liblouis/LiblouisBrailleFilter.java#L72