Train the Avg Perceptron POS model till converged.

As of now, the Avg Perceptron POS model added in #131 gives ~60 per cent accuracy [on CoNLL 2003](https://github.com/Ayushk4/POS.jl/blob/master/valid/CoNLL.ipynb) which is decent for the 30+ classes it addresses. But still is low compared to claimed ~90% accuracy.

We can port weights from other libraries (depending on the licence permissions). Alternatively, we can train those.

But first, it will be better to know precisely the current accuracy Avg Perceptron POS Tagger offers on CoNLL 2000 and GMB as well (CorpusLoaders.jl provides the APIs for these two datasets.)