Allow users to control default variables (maybe via options)

[matt-dray]

Variables are hard-coded into scripts, like default column width, font size, etc. Could provide some sort of config file/allow the user the chance to set options().

The main user-facing functions should remain as simple as possible, given the package-design philosophy (i.e. arguments in new_a11ytable() should just be the components required to build an a11ytable).

[matt-dray]

One option might be cell width. The package currently uses a set width and doubles it if a certain character count is met by the table contents. Might be good to offer users the chance to change these 'default' and 'extended' values, and maybe allow for auto-width (bearing in mind that there will need to be a manual calculation for column A, since auto-width would adjust to the length of the longest of the arbitrary pre-table metadata rows or sheet title in most instances, rather than the content of the first column in the table.