Add support for querying all sheets in a workbook and returning them as result sets

[assmin]

如果一个Excel 有多个Sheet，希望能支持返回多个Sheet 不同的对象, ExcelDataReader 支持返回DataSet 有不同Sheet 的Datatable，但还需要Datatable转list 也比较麻烦，如：有6个sheet ，只需要请求一次io就可以，不用请求6次。

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[michelebastione]

Implementing this feature would be a bit of an architectural challenge, as you yourself pointed out we cannot simply return a collection of objects containing all the data or the advantage of using MiniExcel over other tools would be lost; similarly, returning a collection of enumerables would require some non trivial machinery to allow consumers to read back and forth from any worksheet arbitrarily without creating too much overhead for opening multiple streams concurrently.

The next best thing, which is what I've been working on lately, is a overhaul of the MiniExcelDataReader, which is gonna become OpenXmlDataReader and will implement the NextResult method from IDataReader, allowing for moving between the worksheets sequentially. It's gonna come out in the next v2 preview release.