Float with comma formatting are mis-aligned

When using commas in `floatfmt`, like `,.2f`, alignment is off:
```
In [32]: print(tabulate([("a", 3456.321), ("b", 982.6513)], floatfmt=".2f"))
-  -------
a  3456.32
b   982.65
-  -------

In [33]: print(tabulate([("a", 3456.321), ("b", 982.6513)], floatfmt=",.2f"))
-  -----------
a  3,456.32
b       982.65
-  -----------
```