New syntax request - filtering with Python's 'is in'
Hi all.
With the current version in order to achieve filtering that is reversed to 'pd.Series.isin()' we need to define a function or use a lambda, ie: 

```
df = df[df['city'].apply(lambda x: 'TA' in x if x is not None else False)]
```

I tought it could be a bit easier if we could have a syntax like:

```
df = df['TA' is in df['city']]
```

I believe this will involve overriding `__contains__` in `pd.Series`.
