WIP: Add unit tests for rehydrator.ts

[simon360]

• Adds a .prettierrc file so that the default settings are applied
• Extracts rehydrateChildren to a separate file to aid in later testing
• Exports IOptions and IRehydrator for use in other projects
• Adds unit tests for rehydrator.ts

Closes #15 Closes #32 Relates to #16

To do:

• [ ] cover the case where a rehydrated element has been removed before a render could happen (line 46)
• [ ] organize tests more effectively with describe()