Float format syntax
In for exampe DataFrame.to_csv there is the option "float_format" with the following description:

```
float_format : string, default None

Format string for floating point numbers
```

The currently allowed format in Pandas follows the "old" Python style of '%.2f' % 1.234, so you have to write

float_format = '%.df'.

Being used to 2.7 and 3.x formatting style of Python, I expected that just ':.2f' or even '.2f' would be valid.

To keep up with the new formatting style, should these two last variants (or one of them) also be accepted in Pandas? If not, it should be specified in the documentation, perhaps with an example, that the '%'-style is required.
