Example how to pretrain lm + introduction of config_name

[PiotrCzapla]

I've added ability to limit training set so we can use a test configuration 'multifit_mini_test` that executes in ~20 secs to test that the scripts are working.

Why config_name?

I've added it so we can know what training parameters we should load for the finetune-lm and and classifier. This parameters aren't stored along with a language model, only parameters used to build that model are saved.