API: select_dtypes
related #7308
related #7314

maybe something like this

```
df.get_for_dtypes(types)
```

types could be a list of string/dtypes or a dtype/string

e.g. 

```
'float64'
['float']
['float32','float64','object']
['float',np.dtype('bool')]
['numeric']
['datetime','timedelta']
```

are possible values

pretty easy to implement (just translate the types and filter on the blocks, ala `_get_numeric_data`)
