float_format string error
The float_format string works for single values i.e. float_format='%.1f', but it does not appear to work for a formatted string of columns. For example, If I had four columns with increasing significant digits, separated with tabs:

```
fmt_string='%.1f\t%.2f\t%.3f\t%.4f'
```

Then for some DataFrame X, I should be able to use:

```
X.to_csv('./out.txt',float_format=fmt_string,index=False)
```

But it unfortunately balks at me:

```
TypeError: not enough arguments for format string
```
