Add a Cell class

[jwodder]

Add a public Cell class (not to be confused with the Cell class used internally by the code as of v0.12.0, which will have to be renamed) that wraps and can be passed in place of a cell's content and that takes optional parameters for setting the alignment, padding, etc. for just that cell.

• Cell(None) should be the same as "None", regardless of the value of none_str
• Should Cells be allowed in headers? How would that interact with dict rows?

Some basic test cases for this feature can be found on the feature/cell-class branch.