Apply formatting to tables generated by tablefield module
I haven't installed the module, but from the code it looks like there's an option in the field settings called "Table cell processing" in which you can allow the user to select what Input Format they want. This makes use of Drupal's text formats (Admin > Configuration > Content Authoring > Text formats). You could allow them to use HTML within the cell, or install some alternate Text Format module such as BBCode.