aggregation strategy and offset mapping

[Th3G33k]

Following #750, clean up changes in branch

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• TokenClassificationPipeline aggregation_strategy
• TokenClassificationPipeline start end char
• QuestionAnsweringPipeline start end char
• Tokenizer return_offsets_mapping

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• Similar to Python transformers
• Marked as TODO in the code
• Close Is 'aggregation_strategy' parameter available for token classification pipeline? #633
• Close [Feature request] Return offset mapping using tokenizer #425