Support embedded source maps and definition of a srcmap root

Added a new command line switch (`--maproot`) to define a root for the sourcemap files, this allows to use them on Firefox when loading from a local file (ie: `mjs --maproot file://`).

Also included an option (`--mapfile -`) to embed the generated source map as a comment in the output file. This works great because together with the inclusion of the original source code in the maps allows to have everything bundled with the file loaded by the browser.