"TypeError: v is null" when parsing numbers with exponent

[h3ndrk]

Thanks for the great library! I found a bug in the graph writer:

Consider a graph with a node attribute x=-1.4703333520951247e-16, when writing and reading the same graph, graphlib-dot yields TypeError: v is null in peg$c34:

var dot = require('graphlib-dot');
var graphlib = require('graphlib');

// prepare graph
var graph = new graphlib.Graph({directed: false});
graph.setNode(42, { x: -1.4703333520951247e-16 });

// show output
dot.write(graph); // -> 'strict graph {\n  "42" [x=-1.4703333520951247e-16]\n}\n'

// trigger TypeError
dot.read(dot.write(graph));
// Uncaught TypeError: Cannot read property '3' of null
//     at peg$c34 (node_modules/graphlib-dot/lib/dot-grammar.js:114:28)
//     at peg$parseidDef (node_modules/graphlib-dot/lib/dot-grammar.js:1421:16)
//     at peg$parseaList (node_modules/graphlib-dot/lib/dot-grammar.js:1173:20)
//     at peg$parseattrListBlock (node_modules/graphlib-dot/lib/dot-grammar.js:1093:16)
//     at peg$parseattrList (node_modules/graphlib-dot/lib/dot-grammar.js:1012:12)
//     at peg$parsenodeStmt (node_modules/graphlib-dot/lib/dot-grammar.js:808:16)
//     at peg$parsestmt (node_modules/graphlib-dot/lib/dot-grammar.js:689:20)
//     at peg$parsestmtList (node_modules/graphlib-dot/lib/dot-grammar.js:538:12)
//     at peg$parsegraphStmt (node_modules/graphlib-dot/lib/dot-grammar.js:452:28)
//     at Object.parse (node_modules/graphlib-dot/lib/dot-grammar.js:2372:18)

I think dot.write() produces a non-compliant string (because the ID is not allowed to have a dash -). The parser looks fine.

I'm using graphlib-dot@0.6.4.