Add the ability to process integer-type targets

Currently, the target column can only be a string.
If you try to run with an integer target column, you will get the error:
```
Traceback (most recent call last):
  File "dataset_to_codefiles.py", line 41, in <module>
    print(code[1:-1], file=open(args.train_dir + '/' + str(i) + '|' + y_train.loc[i] + '.py', 'w+'))
TypeError: can only concatenate str (not "numpy.int64") to str
```