Allow compiling to stdout

Low priority, inspired by #625.

Right now the traceur command can't write its output to stdout. Using --out /dev/stdout would work, except that Traceur thinks it's a directory because it doesn't end with .js. Perhaps a better heuristic for detecting compilation to files vs to directories could be chosen? E.g. if it already fs.exists, check if it isDirectory. Or add a new flag such as --stdout.