Add more modify_*() functions

[ddsjoberg]

Add the last functions so users won't need to use modify_table_styling()

• Bolding/Italicizing within the table body
• Missing symbol specification
• Gut any remaining "work" being done in modify_table_styling() and migrate the code to individual functions (I think this only applies to column merging).
• Figure out what to do with .update_table_styling(x). I think it's still helpful to have in modify_table_body(), but do we need it anywhere else?

[ddsjoberg]

At the same time we should gut modify_table_styling() so no real work is being done in this function, and will need to investigate how to handle the use of .update_table_styling(x) in the pkg