Add async support

Author: Jacob Parker

src/__tests__/parser.js

@@ -58,3 +58,15 @@ test('empty selector string', (t) => {
         }).process('').result;
     });
 });
+
+test('async parser', (t) => {
+    t.plan(1);
+
+    t.notThrows(() => {
+        return parser(() => new Promise((res) => {
+            res();
+        })).process('').result.then(() => {
+            t.pass();
+        });
+    });
+});

src/processor.js

@@ -3,6 +3,7 @@ import Parser from './parser';
 export default class Processor {
     constructor (func) {
         this.func = func || function noop () {};
+        this.funcRes = null;
         return this;
     }
 
@@ -15,11 +16,17 @@ export default class Processor {
             options: options,
         });
         this.res = input;
-        this.func(input);
+        this.funcRes = this.func(input);
         return this;
     }
 
     get result () {
+        let isPromise = this.funcRes &&
+            typeof this.funcRes.then === 'function';
+
+        if (isPromise) {
+            return this.funcRes.then(() => String(this.res));
+        }
         return String(this.res);
     }
 }