Allow access to chartsheet items by key
*Created originally on Bitbucket by [CharlieC](https://bitbucket.org/%7B32d660d5-f934-4ce5-a7b0-52750378fae1%7D/)* Currently `wb[sheet.title]` only works for worksheets but there are times when you want access to chartsheets. This will become more important once read-support for charts is added.
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