mpdstats: last_played is documented but not implemented
As pointed out [on the mailing list](https://groups.google.com/d/msg/beets-users/VW0pxtCVZG4/sq9gGsNS9zEJ), the mpdstats plugin (paging @pscn and @kljohann) does not seem to set the `last_played` field, even though the field is described in [the plugin's docs](http://beets.readthedocs.org/en/v1.3.2/plugins/mpdstats.html). Grepping in mpdstats.py for "last_played" shows that doesn't seem to be implemented. We should probably either add it to the plugin or remove it from the docs.

diff --git a/beetsplug/mpdstats.py b/beetsplug/mpdstats.py
index e2f990f44..04355fb31 100644
--- a/beetsplug/mpdstats.py
+++ b/beetsplug/mpdstats.py
@@ -281,6 +281,9 @@ class MPDStats(object):
             'beets_item': self.get_item(path),
         }
 
+        self.update_item(self.now_playing['beets_item'],
+                         'last_played', value=int(time.time()))
+
     def run(self):
         self.mpd.connect()
         events = ['player']
['beetsplug/mpdstats.py']