Add source map generation to bin/jsx

Work in progress for #766

Assuming a directory contains `foo.jsx`, running `jsx -x jsx --source-map . .` will generate `foo.js` and `foo.js.map`.

The generated `foo.js` has the `//# sourceMappingURL=foo.js.map` comment appended.

Current limitations:

- Nasty hack to commoner required to get the output directory.