hyperlinks for pptxGenJs; charts with ext workbook; chart null values

• chore(docs): update readme, more hyperlink tests into single files 2b1e59e
• chore(test): separate hyperlink tests into single files 9383c47
• fix(chart): prevent null values from being written to xml; closes #167 f4708b7
• Merge pull request #170 from singerla/feat-support-chart-with-external-xlsx f8ac403
• feat(test): add chart without workbook to testing pptx 7ef82ee
• feat(chart): support charts without workbook (e.g. linked to external data); fixes #165 9c908b8
• Merge pull request #169 from jrking4/main 2e700b8
• Fix hyperlink rId indexing f2cd69a
• chore(chart): disable copyFiles if no worksheet present e31607e
• chore(xml): remove BOM with TextDecoder before parsing e756e0e

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v0.7.0-0...v0.7.0