Fixing couple bugs

This probably worked before, but with latest version of peg.js (0.8.0) I see parser does not correctly parses basic things.

# Bug 1. 

This graph: `graph 42 {}` will throw an error: `Cannot read property 'map' of null`. This is addressed by replacing `flatten` with build-in `text()` function.

# Bug 2

`graph "G" {}` will have `id` "Gnull", instead of G. This is addressed by checking rest against null in QUOTED_STRING