Support sourcemaps in node execution.

Add dev-dependency on source-map-support.
Add --sourcemaps=memory option to store sourcemap in memory (actually
to not store it as data URL or file on disk.
When sourcemap option selected, supply sourcemap out of memory cache.
Replace Loader.getSourceMapInfo() based on codeUnit lookup with
Loader.getSourceMap(outputName).

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/1475

Closes #1475.