filtering

• filter_by_length
• filter_documents_by_content
• filter_stopwords_arabic
• filter_stopwords_czech
• filter_stopwords_dictionary
• filter_stopwords_english
• filter_stopwords_french
• filter_stopwords_german
• filter_tokens_by_content
• filter_tokens_by_pos
• filter_tokens_by_pos_ratios
• filter_tokens_by_regions