build: Added "uglify-js" as optional compressor

https://github.com/mishoo/UglifyJS2 is a JS-based JS compressor and achieves results close to the closure compiler in simple mode and sometimes even slightly better.

These are the numbers for the `full` build:

Closure: 765.745 Bytes
UglifyJS: 770.581 Bytes

but after `gzip`:

Closure: 191.364 Bytes
UglifyJS: 190.968 Bytes