ftintitle: does not gracefully handle duplicate artist name
Using ftintitle on one of my tracks, it seemed to get tripped up and not be able to fix it. I've tracked it down to being a problem with the fact that the Album Artist's name is in the Artist field twice.

```
Artist: The Roots feat. Talib Kweli / The Roots
Album Artist: The Roots
```

When trying to find the album artist in the artist field, it does a string split using the album artist as a separator. This returns a list with the following values `['', 'feat. Talib Kweli / ', '']`.

The code that tries to find the `feat_part` is then only expecting a two element list, but is instead given a three. It then checks if the `-1th` element, `2` in this case, is blank. If it's not, it extracts the featured artist. 

If it is blank, it goes on to assume the featured artist must be on the left-hand side of the split and checks element `0`.

Both elements `0` and `2` are blank, so no featured part is found.

I've thought of two solutions, but am not sure which one would make more sense
- Attempt to remove duplicate album artists from the artist string before splitting
- Add another/change the current case to iterate over the split parts to find the first non-blank item

Either way, these methods would presumably still leave the trailing slash on the `feat. Talib Kweli /` and add the extraneous trailing slash to the track title. This, I'm not quite sure how to handle if at all.

Thoughts?

diff --git a/beetsplug/ftintitle.py b/beetsplug/ftintitle.py
index 75134ae9c..2e93ad36f 100644
--- a/beetsplug/ftintitle.py
+++ b/beetsplug/ftintitle.py
@@ -83,7 +83,7 @@ def ft_in_title(item, drop_feat):
 
         # Look for the album artist in the artist field. If it's not
         # present, give up.
-        albumartist_split = artist.split(albumartist)
+        albumartist_split = artist.split(albumartist, 1)
         if len(albumartist_split) <= 1:
             ui.print_('album artist not present in artist')
 
['beetsplug/ftintitle.py']