🤗 HuggingFace integration, TextBugger, wider command-line support

big changes:

• load transformers models from the command-line using the --model-from-huggingface option
• load nlp datasets from the command-line using the --dataset-from-nlp option
• command-line support for custom attacks, models, and datasets: --attack-from-file, --model-from-file, --dataset-from-file
• implement attack recipe for TextBugger attack
• add WordDeletion transformation

small changes:

• support white-box transformations via the command-line
• allow Greedy-WIR to rank things in order of ascending importance
• use fast tokenizers behind the scenes
• fix some bugs with the attack Checkpoint class
• some abbreviated syntax (textattack.shared.utils.get_logger() -> textattack.shared.logger, textattack.shared.utils.get_device() -> textattack.shared.utils.device)
• substantially decrease overall TokenizedText memory usage