Implement an option to align final incomplete data chunk

Writing unaligned data causes `EINVAL` (from `write(2)`):

> EINVAL
>  fd is attached to an object which is unsuitable for writing; or the
>  file was opened with the O_DIRECT flag, and either the address
>  specified in buf, the value specified in count, or the current file
>  offset is not suitably aligned.

This pull request implements an opt-in option (to preserve backwards
compatibility) called `align`, that when used in conjunction to `flush`,
will cause the incomplete data chunk to be padded with null bytes to
match the passed chunk size.

For example, in the added test, the chunk size is 4 bytes, and we push
three chunks:

- `12`: 2 bytes
- `34`: 2 bytes
- `5`: 1 byte

The final incomplete data chunk will become `5\0\0\0` (to match the
specified 4 bytes) if the `align` option was enabled.

Signed-off-by: Juan Cruz Viotti <jviottidc@gmail.com>

README.md

@@ -20,6 +20,7 @@ written to this stream must be a `string` or a `buffer`.
 - `chunkSize`: `integer` - Size in bytes of the desired chunks.
 - `opts`
   - `flush`: `boolean` - Optional. Flush incomplete chunk data on stream end. Default is `false`.
+  - `align`: `boolean` - Optional. Pad incomplete chunk data on stream end. Should be used in combination with `flush`. Default is `false`.
   - `encoding`: `string` - Optional. Encoding of String chunks. Must be a valid Buffer encoding, such as `utf8` or `ascii`.
 
 ## Simple Example

index.js

@@ -45,6 +45,13 @@ module.exports = function (chunkSize, opts) {
     var flushFunction;
     if (flush) {
         flushFunction = function (next) {
+
+            if (opts.align) {
+              var remaining = new Buffer(chunkSize - buffer.length);
+              remaining.fill(0);
+              buffer = Buffer.concat([ buffer, remaining ], chunkSize);
+            }
+
             this.push(buffer);
             next();
         };

test/flush.js

@@ -38,4 +38,26 @@ test('Test flush option', function (t) {
     chunkerFlush.write('56');
     chunkerFlush.end();
 
-});
+});
+
+test('Test align option', function (t) {
+    t.plan(1);
+
+    var optsFlushAlign = {
+        flush: true,
+        align: true,
+        encoding: 'utf8'
+    }
+
+    function checkFlushAlign(data) {
+        t.equals(data, '12345\0\0\0', 'Received flush data');
+    }
+    var chunkerFlushAlign = Chunker(4, optsFlushAlign);
+    var concatStreamFlushAlign = concat(checkFlushAlign);
+    chunkerFlushAlign.pipe(concatStreamFlushAlign);
+    chunkerFlushAlign.write('12');
+    chunkerFlushAlign.write('34');
+    chunkerFlushAlign.write('5');
+    chunkerFlushAlign.end();
+
+});