Feature: New object type for containing all options for single cells

I'm looking to make a new object type to cover all cases of writing to the excel file. The goal would be that all available formatting and other options (comments, hyperlinks, etc.) could be covered by this object and that it could simplify the writing to the excel files themselves. In general, I think it would look like a list S3 object, and it would have many `as_writexl_cell()` generic functions. My initial list is that it would contain: * value: the value itself (which could be any class such as character, numeric, date, ...) * By storing the object as a list, we could enable mixed-type columns such as numeric in some cells and text in others within one column. In general, any cells within a column could then have different features and types (such as some numeric values followed by formula followed by text or hyperlinks). * format: mapping to all the options in https://libxlsxwriter.github.io/working_with_formats.html * formula: similar to the current handling, but treated as separate from the value * hyperlinks: This is the cell-level hyperlink * comments: mapping to all the options in https://libxlsxwriter.github.io/working_with_comments.html Related to this would be color handling functions. My goal is that all cell writing would occur through this object. This would also not be a breaking change as existing objects would be maintained. @jmcnamara Are there features that this new cell-object type should have based on features available in `libxlsxwriter` that I have missed?