Add ability to plot area kind with "steps" drawstyle
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
import matplotlib.pyplot as plt
df = pd.DataFrame()
df['x'] = range(0,100,5)
df['y'] = range(0,1000,50)
df = df.set_index('x')

# line plot 
df.plot.line()
plt.savefig('line')

# stepped line plot
df.plot.line(drawstyle="steps")
plt.savefig('line-steps')

# area plot
df.plot.area()
plt.savefig('area')

# Feature Request: stepped area plot
df.plot.area(drawstyle="steps-mid")
plt.savefig('area-steps')

# Alternative solution, use bar plots.
df.plot.bar(width=1.0)
plt.savefig('bar')

```
#### Problem description

It would be nice to be able to create a "stepped" area plot.  Similar to a line(drawstyle="steps"),  but with the area under the line filled in:

 **df.plot.line(drawstyle="steps")**
![image](https://user-images.githubusercontent.com/1243301/68348895-2f896900-00b0-11ea-99d6-81da5d240442.png)


As a work around,  I have been plotting a `bar` plot with `width=1.0`:  

**df.plot.bar(width=1.0)**
![image](https://user-images.githubusercontent.com/1243301/68348962-65c6e880-00b0-11ea-9033-5be571ef04ff.png)

however this has issues of its own:

1. It is much slower with lots of data  
2. Will have to limit the xaxis labels,  by default it will generate a label per bar.
3. It doesn't interact well with line plots on a secondary y-axis 

Alternatively,  I think could manually resample the data,  but it would be really nice to have similar `drawstyle='steps'`

#### Expected Output

I would expect a plot similar to this:

**df.plot.area(drawstyle="steps")**
![image](https://user-images.githubusercontent.com/1243301/68348716-92c6cb80-00af-11ea-9606-16c4a9f3bff1.png)

