Support generateSourceMaps and preserveLicenseComments when using uglify2

Support using the generateSourceMaps and preserveLicenseComments options
together when using uglify2. Uses the preamble option of uglify2 to
include the license comments and have the source maps "adjust for its
presence".

I'm not sure if I should have implemented tests of this functionality or if I should have documented it otherwise, but then the approach might at least be useful.