Hybrid word-character decoder

[Waino]

Implement a two-level decoder following [1].

First a word-level decoder produces a sequence that may contain <UNK> symbols.
Next, these <UNK> symbols are filled in using a character-level decoder.

[1] Luong, Minh-Thang, and Christopher D. Manning. "Achieving open vocabulary neural machine translation with hybrid word-character models." arXiv preprint arXiv:1604.00788 (2016).
http://arxiv.org/pdf/1604.00788