Allow inSourceMap option to be a generated JSON source map

It seems a little silly to have to write a generated source map to a file or even to a string just to use it right away in UglifyJS.  I was using the long hand API and manually using the `source_map` option to `ast.print_to_string` but it seemed a little unnecessary so I added the ability for `inSourceMap` to be a generated JSON source map as well as a string filename.  This should solve issue #123 as well.