Add `na=` parameter to str.isalnum() and other tests
I have a use case where I want to select all the rows in a column that are not numbers:

`lc.loc[lc.id.str.isalpha()]`

Some rows in the `id` column are nan so I get a value error:

`ValueError: cannot index with vector containing NA / NaN values`

The work around is to use:

`lc[lc.id.str.contains('[A-z]', na=False)]`

 I can provide a reproducible example if desired. 

It would be nice if there were an na parameter in the string tests as well. E.g., `lc.loc[lc.id.str.islapha(na=False)]`.

On a related note, the the pattern `fillna(False)` to avoid this error doesn't seem to be documented well.  My understanding is that this is just an arbitrary value and can easily have been True or 'apple'. Is this correct? 

I'd be happy to add this to the documentation if my understanding is accurate. 
