Avoid a max call stack size for documents with many relationships

Create a function to check for invalid values (#38)

Invent a recursive function that checks against NaN input in <Table> and <Cell> props

Support themes by implementing `ThemeXml`, solve a DOCX inconsistency…

… around default fonts

Add `defaultTabStop` setting (#36)

Tabs in paragraph properties (#35)

Support paragraph-level tab specifications

Added extra relationships required for 2020 and legacy template

Forgot a semicolon.

Fix a formatting issue

Adopt the use `ContentTypesXml` and implement support for SVG in `Image`

Support shading on <Text>