No source map option

While I personally enjoy source maps, it is certainly possible that someone could just want to minify and skip the source map piece, perhaps for something like a production build.

At the moment, this is not possible in minifyify, it either has to have the source map inline, or have a comment pointing to an external source map. This pull request makes it such that if you pass `false` as the value for the `map` option, there is no source map present.