Add support for sourcemaps.

This combines various PRs that already exist, but provide a better end result. Basically, we always want to create sourcemaps, which requires `opts.outSourceMap` be set in  uglify, not just when the input file has a sourcemap (previous behavior). We also want to allow the user to disable sourcemaps entirely. This combines and improves upon (due to combined behavior) https://github.com/hughsk/uglifyify/pull/18 https://github.com/hughsk/uglifyify/pull/21 and https://github.com/hughsk/uglifyify/pull/25