Formalise the user-input 'content' object about a11ytables

[matt-dray]

I think the answer is to make the input object an S3 class so the strictness can be handled; we can develop a sensible print method for it; and so that the function that converts the input to a Workbook-class object can refuse objects that don't have the class.

In other words, the process for the user should be relatively simple:

1. Get their data and supporting information into a 'strict' S3 object (suggested class name: 'a11ytable')
2. Pass that object to a single function that generates an output Workbook-class object, ready for tweaking/writing (made possible with #16)
3. Tweak/write with {opensxlsx}