Enhancements to cut , memos, and ParserConfig

• [contexts] reinstate cache pruning on cut (#379)
• [docs] Document ParserConfig configurations (#378)
• [config] Move ParserConfig to its own tatsu.parserconfig module
• [parrserconfig] remove unused attribute owner
• [grammars] always set a name for the grammar
• [tokenizing] add the unimplemented line_index()
• [parserconfig] add memoization parameter/option and directive (#377)
• [parsing] remove the FailedCut exception and the associated logic (#376)