Create output file by gulp, not `psc` command

The `output` option given to `psc` function should be passed to gulp,
not `psc` command. If it was passed to `psc` command, the file
will be created and gulp won't receive any input stream from this function.
Update README
Using omit to filter the output key
Merge branch 'fujimura-create-output-file-by-gulp-not-psc-command'