* Fixed issue #539 Fixed error opening excel file created in encoding different from UTF-8, added logging of possible errors when decoding XML if the function does not provide exit with an error

* Added test for CharsetReader

* Fixed #discussion_r359397878

Discussion: https://github.com/360EntSecGroup-Skylar/excelize/pull/540#discussion_r359397878

* Fixed go fmt

* go mod tidy and removed unused imports

* The code has been refactored

workBookRelsWriter, drawingRelsWriter into relsWriter;
drawingRelsReader, workbookRelsReader, workSheetRelsReader into relsReader;
addDrawingRelationships, addSheetRelationships into addRels

Fix potential memory leak where zw is not Close() when an error occurs.

add error return value

exported functions:

GetMergeCells
ProtectSheet
UnprotectSheet
UpdateLinkedValue
GetMergeCells
SetSheetVisible

inner functions:

workSheetReader
copySheet

* add new exported function to get Excel file content buffer

* refactor the WriteTo function

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

Extract a WriteTo() method (see io.WriterTo) that exposes the count of bytes
written and rewrite Write() to use it.

- go test and godoc has been updated

- API changed, use worksheet name instead of "sheet" + index, related issue #25, #43, #47, #51, #89, #101, #116 and #120.
- go test updated

- Function `formattedValue()` performance improvement by avoid repeating deserialization, relate issue #64;
- Make function `ToAlphaString()` exportable, relate issue #63

- Doc for function `GetRows()` updated, relate issue #43;
- Fix doc typo in `xmlContentTypes.go`;
- Default template updated;
- Readme updated;
- go test updated

- Optimize deserialization operations;
- README updated, add go version required notice

- Add escape characters of sheet name;
- Update go test and fix typo