Add option to dump only leafs in --watch

While working with many files that are combined under single asset I noticed how slow it is when script has to dump 10+ leafs when only one changes. The new option allows to dump only leafs. And, since I'm using assetic in debug mode without controllers, that's exactly what I need. I think this should benefit everyone who's developing javascript heavy applications with more than 5 files combined. Especially when using compilers (coffeescript, less etc).