read_fwf with passing converters and na_values at the same time
Duplicate of #13302

---

When reading via read_fwf and passing both na_values and converters at the same time, na_value detection does not work:
#### Code Sample, a copy-pastable example if possible
Assume a fwf-style text file like this:
> A  B  C
> 10 -1 10
> 10 10 10

```python
In: read_fwf(file_path, converters={'B':lambda x: int(x)/10.}, na_values={'B':-1})
Out:
>     A    B   C
> 0  10 -0.1  10
> 1  10  1.0  10

```
#### Expected Output
```
>     A    B   C
> 0  10  NaN  10
> 1  10  1.0  10
```
It would be nice if this could be handled behind the scenes.
In pandas.io.parsers._clean_na_values, float representations of the na_values are produced by _floatify_na_values. Maybe passing the converters to this function might be a solution?


#### Output of ``pd.show_versions()``
<details>
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.10.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 23 Stepping 10, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.19.2
(...)
</details>
