Allow opening multiple destinations.

My changes converts string `dest` to a single item array to be backward compatible. If you pass an array of destination in `dest` config in Gruntfile, it will loop on that array and open those destinations one by one.

Here is the screencast ![screencast](https://d1ro8r1rbfn3jf.cloudfront.net/ms_40046/JcsuzflqIpx7Xr23CDXebohuJoFONP/pr.gif?Expires=1413152315&Signature=nBDkfpK2gx9xpm13jApEix2jKEpAeiJg3X9M3rCmXdQPoslK5Wu0xUoaIWmCFZeaIKLvtoKsPrlW2Oz0x4KQyOUrqWAOZ3Krp1WzMsprE9xla9lLGLEfqtJ-PsBWlbrmkunVzAxoKWx-funqVl1kq~2h-s0Ymwrqv0kKTfXTn9lwwGFj8AdMxqL1Miku-aJuGKYm7dkD-CDCKbKVgkrx9nDHNZrzxp9Ie1hsS57a8ZnO93k-D0~N40PfE73SGfevwpA1YOHeun7-Su-RrfHr9F~Wlgs8Hyr~7krCb~5rri49NSmr0mlbvj5Gg5FSnBP6l5qbJXXcwqT5-eBQQGbd4A__&Key-Pair-Id=APKAJHEJJBIZWFB73RSA)

In any case gif link expires, here is the [original link](http://monosnap.com/file/VNlVZyp09Oil3pxfDBi8te7mu67K6D) to screencast.