Patch

@imlucas 
1. better bin/mongodb-datasets.js: now can do `cat schema.json | mongodb-datasets.js --size 10 > out` to write the result to file. All supported commands are:
* `--size=<number>` - the size of generated docs
* `--populate [--uri <db uri> --collection <name>]` - whether to populate the result to mongodb, default is false
* `--raw=true/false` whether to format the json docs, default is true
2. made Populator also a Transform stream to support chaining more pipes after it
3. enhanced the scoping feature to support access to parent-level variables by `this._$parent.<var>`