add option to pass language directly

[na50r]

Instead of relying on googletrans, it would be convennient if I could do:

aligner = Bertalign(
        src=en, 
        tgt=de,
        src_lang='en',
        tgt_lang='de',

Language only matters for Sentence Splitting and it is not wrong to assume that people who use this tool likely already know the source and target language, so there is no harm in allowing them to pass it in directly.