support UglifyJS screw_ie8 option via screwIe8 property

By default UglifyJS tries to be IE-proof, passing screw_ie8 makes it not care about IE<9 quirks like leaking named function expressions names to the outer scope. If a project doesn't support IE8, passing this flag can reduce the output file size.

This pull request adds a `screwIe8` option to the `uglify` grunt task which is then passed to the UglifyJS mangler & compressor as `screw_ie8`.