This adds the ability to send any command to a player, usefull for playl...

...ists and other plugins.  This allows triggering of playlists and other features in a squeeze server.  For example on my system I use both the spotify and pandora plugins, now I can trigger playing a playlist either with a static item bnding like so
```
Switch sq_office_pandora	"pandora" { squeeze="office:command:pandora playlist play item_id:ca30491a.0.14", autoupdate="false"
```
or pass it in via a text item from say a switch (or action or rule)

```
String sq_office_command	{ squeeze="office:command", autoupdate="false" }

Switch item=sq_office_command mappings=["pandora playlist play item_id:ca30491a.0.14"="Grand National Radio", "pandora playlist play item_id:de875420.0.14"="Broken Bells Radio"]
```