fixes #363 h/t @Mior note: @sheetjsdev rewrote implementation, original PR author used
- clarify usage of Props and Custprops (fixes #274 h/t @michahell) - SYLK from js-harb - DIF from js-harb - HTML empty string bug fix
- chartsheets are now stored as sheets with "!type" set to "chart" - fixed Strict XML Workbook Relationships - stubbed support for dialog and macro sheets - removed legacy XLS `!range` field
