Default behavior of plot.bar changed to plot a different color for each bar
#### Code Sample, a copy-pastable example if possible
**BEFORE**
```python
df_india.groupby('sector').id.count().sort_values().plot.bar();
```
would result in a bar plot with all bars having the same color

**AFTER** version 0.20
```python
df_india.groupby('sector').id.count().sort_values().plot.bar(color='cornflowerblue');
```
I need to explicitly provide a color, else each bar is colored differently

Minimal reproducible code:

```
import pandas as pd
df = pd.DataFrame({'account-start': ['2017-02-03', '2017-03-03', '2017-01-01'],
                   'client': ['Alice Anders', 'Bob Baker', 'Charlie Chaplin'],
                   'balance': [-1432.32, 10.43, 30000.00],
                   'db-id': [1234, 2424, 251],
                   'proxy-id': [525, 1525, 2542],
                   'rank': [52, 525, 32],
                   })
df.client.value_counts().plot.bar()
```

Output:

![download](https://user-images.githubusercontent.com/3866405/38206924-d34dffcc-36c9-11e8-866c-57364f90042b.png)


#### Problem description
Why was the behavior of Series.plot.bar changed to plot bars with different color? Visually these colors add nothing to the plot as different colors should only be used when they correspond to differences of meaning in the data.

Why is the default behavior to provide an unnecessarily visually overwhelming graph? It took me some time to realize why my bars suddenly started acting strangely. Now, I pass color='cornflowerblue' to get all my bars the same pleasant hue.

Reference for visual appeal and the use of colors: http://www.perceptualedge.com/articles/visual_business_intelligence/rules_for_using_color.pdf

Would it be possible to revert to the pre-0.20 behavior?

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

<details>

[paste the output of ``pd.show_versions()`` here below this line]

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.4.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.0-5-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: C.UTF-8
LANG: C.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.22.0
pytest: 3.2.1
pip: 9.0.1
setuptools: 38.5.2
Cython: 0.26.1
numpy: 1.14.2
scipy: 1.0.0
pyarrow: 0.8.0
xarray: 0.10.1
IPython: 6.1.0
sphinx: 1.6.3
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2018.3
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.2
feather: 0.4.0
matplotlib: 2.1.0
openpyxl: 2.4.8
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.1.0
bs4: 4.6.0
html5lib: 0.9999999
sqlalchemy: 1.1.13
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: 0.6.0
</details>
