ENH: Dataframe isin2
fixes #4211, an alternative (on top of) #4237

DataFrame isin method:

```
In [11]: df = pd.DataFrame([['a', 'a', 'c'], ['b', 'e', 'a'], ['c', 'a', 'f']], columns=['A', 'A', 'B'])

In [12]: df
Out[12]:
   A  A  B
0  a  a  c
1  b  e  a
2  c  a  f

In [13]: df.isin(['a'])
Out[13]:
       A      A      B
0   True   True  False
1  False  False   True
2  False   True  False

In [14]: df.isin({'A': ['a']})
Out[14]:
       A      A      B
0   True   True  False
1  False  False  False
2  False   True  False

In [15]: df.isin({0: ['a']}, iloc=True)
Out[15]:
       A      A      B
0   True  False  False
1  False  False  False
2  False  False  False
```

cc @TomAugspurger 
