Added two exported functions `SetWorkbookPrOptions` and `GetWorkbookPrOptions` to support setting and getting the code name property of the workbook
Re-order fields of the workbook properties group to improve the compatibility
Go Modules dependencies upgrade
Put workbook related operating in new `workbook.go` source code
Library introduction docs block updated

Rename exported constants `NameSpaceDublinCoreMetadataIntiative` to `NameSpaceDublinCoreMetadataInitiative`

Go 1.15 and later required, #65 fn: IMABS, IMCOS, IMCOSH, IMCOT, IMCSC, IMCSCH, IMEXP, IMLN and IMLOG10

typo fixed

- RGBA, HSL color convert has been added;
- go test updated