When dest option is a directory, uglify each input file as an individual

Just a small enhancement, one that I've wanted for a while so went ahead and wrote this code to do it. In my case, I have a folder comprised of lots of small scripts that I wanted to compress as individuals. (ie. not concatenated) The list was long enough that manually defining each input file was tedious/error-prone.

The change I made works like this: when the `dest` option is a directory, the src files are uglified as individuals, rather than as a group.