Added 'Share' Command

Not sure if you want this functionality, but I am using it quite a bit so I thought I'd share. I've added `share` as a command, which displays, as well as copying to clipboard, the share URL for the current track. It works like this:
```
$ spotify share
Share URL: http://open.spotify.com/track/01PdqaCvij7ryXod6nbtyQ
```

It also copies the share URL to the clipboard using `pbcopy`.