Plugin simplified, new functionality added.

Header row is now a selectable option when sorting, not a per file setting.  I didn't like how it was stored in the User's preferences.
Ascending vs. Descending sort is also selected when sorting, not a separate command.
Sorting attempts to sort numerically before falling back to text sort.
Added Insert column and Delete column features.

Notable TODO items:
+ The code which detects the selected column doesn't handle quoted values in the CSV table, this is not good.
+ After reformatting the document, the cursor position should be restored to the cell that was selected.
+ Some syntax for embedding cells to be evaluated.