Add support for root-level arrays

[davilimap]

Currently, the root-level element cannot be an array, the transformation will fail. First off, I think we need to add better error handling to improve the user experience. Then, look into supporting this without breaking transformations.

See: microsoft/slow-cheetah#92

[tsahi]

https://www.json.org/json-en.html specifies that JSON must start with curly braces.