Allow arbitrary pre-table meta information

[matt-dray]

Should users be able to provide arbitrary content above the table, i.e. via an argument(s) to new_a11ytable()?

At the moment, users of the package can only provide information about very specific things: the title, table count, warning of notes, explanation of blanks (upcoming v0.1) and data source (arguably there should be more defined possibilities, like an explanation of shorthand).

I think it's positive to be restrictive; the whole point is that we want people to adhere to the guidance and not write anything they like above each table. However, this isn't great for situations where there's a genuine user need for specific additional information.

[matt-dray]

Might be easier to supply arbitrary content with an approach like #65, but would still need functions to interpret the YAML and insert each item and the table correctly into each sheet.

[matt-dray]

This could be an argument called custom in create_a11ytable(). It would be a list of character vectors; one list-element per sheet, as many vector elements as the user wants. Each custom vector-element will appear on its own row above the table. No special styling will be applied (the same as for the non-sheet-title text).

Will need to adjust the functions that count pre-table meta rows to take account of an arbitrary number of custom rows, so that tables are inserted correctly.

[matt-dray]

For backwards-compatibility—and because they're important features to have in a sheet—arguments sources and blank_cells should be retained in create_a11ytable(), even though a user could just ignore them and supply this information as custom rows.

[matt-dray]

The arbitrary rows should support hyperlinks, as per #47.

[matt-dray]

The custom_rows argument should come after the sources argument in create_a11ytables(), maybe?

At the moment I've put custom_rows before, so that the order of the arguments and the order of the the pre-table rows in the output will match: title, number of tables (generated automatically), presence of notes (generated automatically), meaning of blank rows, custom rows, source.

But if someone has used create_a11ytables() before v0.3 then they may have provied arguments by place and their code will error if they update the package.

Most users probably have named their arguments in create_a11ytable(), though. So maybe this is a worthwhile, small breaking change.