support json filename extension.

After generate json, the output filename extension was .js.

However, we might think the output filename extension is .json after compile livescript to json. So I change replaceExtension with json configuration.

```
gulp.task 'json', ->
  gulp.src 'json/*.ls'
    .pipe gulp-plumber!
    .pipe gulp-livescript({+json,+bare}).on 'error', gutil.log
    .pipe gulp.dest "json/"
```

It would output file with .json filename extension.