Use 'spawn' instead of 'exec' to call hilbert-cli

[elondaits]

Exec uses a fixed-size buffers, so it can fill up when calling a cfg-query command. Spawn uses streams instead of a buffer, so it does not have a (practical) limit.

[elondaits]

Relevant:

http://www.hacksparrow.com/difference-between-spawn-and-exec-of-node-js-child_process.html