Add flat and flatMap.

lib/index.js

@@ -172,6 +172,28 @@ const Channel = function(length = 0) {
         return output;
       },
 
+      flat: depth => {
+        const output = Channel();
+
+        (async () => {
+          await readOnly.forEach(async value => {
+            if (Channel.isChannel(value)) {
+              const input = depth > 1 ? value.flat(depth - 1) : value;
+              await input.forEach(output.push);
+            } else {
+              await output.push(value);
+            }
+          });
+
+          await output.close();
+        })();
+
+        return output;
+      },
+
+      flatMap: (mapperFunction, thisArg) =>
+        readOnly.map(mapperFunction, thisArg).flat(),
+
       forEach: async (callbackfn, thisArg) => {
         for (;;) {
           const value = await readOnly.shift();