Allow input source map

[yuri-karadzhov]

As it's no possible to concat to one file with no modules it's good to support input source map. So it will be possible to concat first (and get a source map) then traceur with input source map and get compilled file with new source map.

[sindresorhus]

See gulpjs/gulp#356 and https://github.com/floridoo/gulp-sourcemaps#plugin-developers-only-how-to-add-source-map-support-to-plugins

[blittle]

This will be huge for us in adding traceur to our build pipeline :)

[sindresorhus]

See #24