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)
