Would like to be able to process a stream readonly

[breynolds]

In Azure, we can store files in blob storage and then process them. To process a file that is stored as a file, we can request a stream for reading, but then to use that stream with FastExcel(Stream stream) there is no read-only option. The readonly stream throws an exception when FastExcel tries to open it with read/write access. Would have wanted to have an option to create a FastExcel instance with a constructor such as FastExcel(Stream stream, bool readOnly = false) so we could simply read the excel stream.