Feature request: vectorized sort on string split results
#### Problem description

I'm searching without success a way to sort the results of `pd.Series.str.split`.

I was expecting a str method, as we have it for example to access slices of the results, but it seems that it doesn't exists (tried `sorted`, `sort`, `sort_values` and googled).

As a workaround, I'm using `apply` on the `split` result, but it is not vectorized, so I guess that it can be made faster with vectorized methods.

Unfortunately, I don't see how we can implement the additional arguments of the python `sorted` to that vectorization, especially the `key` and  `reverse` arguments. It would be great to have that possibility as well.

Thanks!

#### Code Sample

```python
import pandas as pd

###########
# Desired #
###########
pd.Series(["a b:d c"]).str.replace(':', ' ').str.split(' ').str.sorted.str.join(' ')
# ---------------------------------------------------------------------------
# AttributeError                            Traceback (most recent call last)
# <ipython-input-2-6fb06dcfcab2> in <module>()
# ----> 1 pd.Series(["a b:d c"]).str.replace(':', ' ').str.split(' ').str.sorted.str.join(' ')
# 
# AttributeError: 'StringMethods' object has no attribute 'sorted'

##############
# Workaround #
##############
pd.Series(["a b:d c"]).str.replace(':', ' ').str.split(' ').apply(sorted).str.join(' ')
# 0    a b c d
# dtype: object

```

#### Used versions

* Python 3.6.1 (Anaconda)
* Pandas 0.20.3