Fixes#295
If a row should be written with a custom style, the handling of empty cells should change.
Instead of being skipped entirely, empty cells will be applied the custom style, if this style has custom background color or borders.
If not, then the cell definition can still be skipped.
This fixes issues when something went wrong on reader/writer init and the developer wants to close the reader/writer.
The file handle may not be defined so we need to add a check for it, before actually using it.
Since cells are referenced by their coordinates (A2, B4...), it is not necessary to write empty cells.
This will reduce the final size of the generated XML and therefore XLSX file.
Added ODS writer
Refactored XLSX writer to abstract some pieces into an abstract multi-sheets writer
Created an abstract style helper
Moved shared components around
Added top level methods on the Writer:
- addRowWithStyle()
- addRowsWithStyle()
Added a style builder, to easily create new styles.
Each writer can specify its own default style and all styles will automatically inherit from it.
For now, the style properties supported are:
- bold
- italic
- underline
- strikethrough
- font size
- font name
- wrap text (alignment)
