add header detector.

src/index.js

@@ -2,7 +2,7 @@ const assert = require('assert');
 import querystringLookup from './lookups/querystring';
 import pathLookup from './lookups/path';
 import cookieLookup from './lookups/cookie';
-// import headerLookup from './lookups/header';
+import headerLookup from './lookups/header';
 // import sessionLookup from './lookups/session';
 
 function getDefaultsOpt() {

src/lookups/header.js

@@ -0,0 +1,56 @@
+export default {
+    name: 'header',
+
+    lookup(ctx, options) {
+        let found;
+        if (typeof ctx === 'undefined') {
+            return found;
+        }
+        let headers = ctx.headers;
+        if (!headers) {
+            return found;
+        }
+
+        let locales = [];
+        let acceptLanguage = headers['accept-language'];
+
+        if (acceptLanguage) {
+            let lngs = [];
+            let i;
+
+            // associate language tags by their 'q' value (between 1 and 0)
+            acceptLanguage.split(',').forEach(function(l) {
+                let parts = l.split(';'); // 'en-GB;q=0.8' -> ['en-GB', 'q=0.8']
+
+                // get the language tag qvalue: 'q=0.8' -> 0.8
+                let qvalue = 1; // default qvalue
+
+                for (i = 0; i < parts.length; i++) {
+                    let part = parts[i].split('=');
+                    if (part[0] === 'q' && !isNaN(part[1])) {
+                        qvalue = Number(part[1]);
+                        break;
+                    }
+                }
+
+                // add the tag and primary subtag to the qvalue associations
+                lngs.push({ lng: parts[0], q: qvalue });
+            });
+
+            lngs.sort(function(a, b) {
+                return b.q - a.q;
+            });
+
+            for (i = 0; i < lngs.length; i++) {
+                locales.push(lngs[i].lng);
+            }
+
+            if (locales.length) {
+                found = locales;
+            }
+        }
+
+
+        return found;
+    }
+};