Add a command for evaluation

I would like to create a new command pylaia-htr-evaluate to compute CER/WER.

• Command:

pylaia-htr-evaluate --config evaluation_config.yaml

• Configuration file:

eval:
   labels:
      train: path/to/train_labels.txt
      val: path/to/val_labels.txt
      test: path/to/test_labels.txt
   predictions:
      train: path/to/train_predictions.txt
      val: path/to/val_predictions.txt
      test: path/to/test_predictions.txt

• Output formatted in markdown:

Set	CER (%)	WER (%)
train	2.40	8.10
val	7.45	19.75
test	6.55	18.2

Ported from https://github.com/jpuigcerver/PyLaia/issues/72