ENH: option for groupby.hist to match bins
- [x] closes #22222
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Example code and result:

```python
%matplotlib inline
import numpy as np
import pandas as pd

def test_hist_bins_match():
    # GH-22222
    N = 100
    bins = 5

    np.random.seed(0)
    df = pd.DataFrame(np.append(np.random.randn(N), np.random.randn(N) / 10),
                   columns=['rand'])
    df['group'] = [0] * N + [1] * N
    g = df.groupby('group')['rand']
    bin_range = np.linspace(df['rand'].min(), df['rand'].max(), bins + 1)

    ax = g.hist(bins=bins, alpha=0.7, equal_bins=True)
    both_hists_max = g.apply(lambda x: max(
        np.histogram(x, bins=bin_range)[0])).max()
    assert ax.iloc[0].get_ylim()[1] >= both_hists_max

test_hist_bins_match()
```

![image](https://user-images.githubusercontent.com/26352146/43753017-add30f56-99d1-11e8-9b20-fecbdfa20fb2.png)
