enhancement: optionally pass params for hyperparam line search in trainer.train() methods

[amn41]

for the ner_trainer and text_categorizer_trainer classes, the train method offers no control over the window (eps) or max_iter of the find_max_single_variable function.

I would propose to make those optional args to the train methods.

[davisking]

Sure, that's fine with me so long as it's clear that users are expected to not need to modify those parameters. So such a PR would be welcome :)