to_dict('index'): header=False
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd

df = pd.DataFrame(
    {'col1': [1, 2], 'col2': [0.5, 0.75]}, index=[400, 800])

df.to_dict('index', header=False)
# {400: [1, 0.5], 800: [2, 0.75]}
```
#### Problem description

It would be cool if the `DataFrame` [`.to_dict()`](https://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.to_dict.html) functionality would support a new option `header=` just as the [`.to_csv()`](https://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.to_csv.html) export.

In my specific case, the dataframe as above is a matrix with a time series as index and columns of bins.
I would love to export it to a dict with only the `index` as key but a plain 1D array as values.

Currently, the `index` export will put a dict of dict, namely the first index being the `index` and the inner index to the `column names`:

```python
df = pd.DataFrame(
    {'col1': [1, 2], 'col2': [0.5, 0.75]}, index=[400, 800])

df.to_dict('index')
# {400: {'col1': 1.0, 'col2': 0.5}, 800: {'col1': 2.0, 'col2': 0.75}}
```

#### Expected Output

My current work-around looks like this:
```python
iteration=df.index.values
dict(zip(
    iteration,
    df.loc[iteration].as_matrix()
))
# {400: array([1. , 0.5]), 800: array([2.  , 0.75])}
```

#### Output of ``pd.show_versions()``

```
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.0-5-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_US.utf8
LOCALE: de_DE.UTF-8

pandas: 0.22.0
pytest: None
pip: 9.0.1
setuptools: 36.2.7
Cython: None
numpy: 1.14.0
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 6.1.0
sphinx: 1.6.3
patsy: None
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: None
tables: 3.3.0
numexpr: 2.6.1
feather: None
matplotlib: 2.1.0
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.5.3
html5lib: 0.999999999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
```