Log undefined variables (MadLittleMods#69)

Fix MadLittleMods#31

Author: pixeldrew

index.js

@@ -234,7 +234,7 @@ module.exports = postcss.plugin('postcss-css-variables', function(options) {
 				ruleToWorkOn.nodes.slice(0).forEach(function(node) {
 					if(node.type === 'decl') {
 						var decl = node;
-						resolveDecl(decl, map, opts.preserve);
+						resolveDecl(decl, map, opts.preserve, logResolveValueResult);
 					}
 				});
 			});

test/test.js

@@ -199,6 +199,9 @@ describe('postcss-css-variables', function() {
 						var colorDecl = fooRule.nodes[0];
 						expect(colorDecl.value).to.be.a('string');
 						expect(colorDecl.value).to.be.equal('undefined');
+
+						expect(result.warnings()[0].type).to.be.equal('warning');
+						expect(result.warnings()[0].text).to.be.equal('variable --missing is undefined and used without a fallback');
 					});
 				});
 		});