Performance enhancements, remove redundant XML element checking logic. Relate issue #29. The benchmark report of the current version of this library is shown on the wiki page.

- Partial logic performance optimization, use pointer reference instead of a pass the variable value;
- Add comments for content types struct definition;
- Update go test `TestSetBorder` section

- Function parameter code is simplified;
- Fix element `Tint` value parsing error in worksheet;
- Update go test

- Improved performance when reading large files, call Token to read tokens one by one instead Unmarshal. Related issue #20 ;
- Fix go test typo;
- Update README

Make functions: `SetCellValue`, `SetCellInt`, `SetCellHyperLink`, `SetCellFormula`, `GetCellValue` and `GetCellFormula` to support the merged cells.

- Character limits for cells added;
- Update go test and fix typo

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

- go test updated;
- fix typo and `gofmt -s` formatted

SetCellStr escapes the value in a cell so the excel sheet cannot use the value in formulas.

SetCellDefault accepts a string value and sets it to a cell as a raw value without escaping it

- New functions `GetActiveSheetIndex`, `GetSheetName` and `GetSheetMap` added.

- Update workbook and sheet relationships and self-close tag replacement hack functions

- Fix issue XML tag `headerFooter` and `sheetPr` element self-close errors cause file corruption;
- Fix issue `Section` and `Pane` element order make file corruption in some case;
- Change sheet `rId` calculation method in `/xl/workbook.xml`, fix makes file corruption in some case;
- Compatibility improved: add `xlsxTabColor` struct and some XML element for worksheet

Fix hyperlink missing after save issue and update completion row element logic to enhance compatibility.

Open file error return added and UpdateLinkedValue function added to fix linked values within a spreadsheet are not updating.

- Optimisation code use fmt package
- Update README
- Remove useless function