Support O_APPEND flag in *MemMapFs.Openfile

To allow opening an existing file in append mode.

fs_test.go

diff --git a/fs_test.go b/fs_test.go
index ab667e9..bdbdc7e 100644
--- a/fs_test.go
+++ b/fs_test.go
@@ -75,12 +75,30 @@
 		path := testDir + "/" + testName
 		fs.MkdirAll(testDir, 0777) // Just in case.
 		fs.Remove(path)            // Just in case.
+		defer fs.Remove(path)
 
-		f, err := fs.OpenFile(path, os.O_CREATE, 0600)
+		f, err := fs.OpenFile(path, os.O_RDWR|os.O_CREATE, 0600)
 		if err != nil {
 			t.Error(fs.Name(), "OpenFile (O_CREATE) failed:", err)
 			continue
 		}
+		io.WriteString(f, "initial")
+		f.Close()
+
+		f, err = fs.OpenFile(path, os.O_WRONLY|os.O_APPEND, 0600)
+		if err != nil {
+			t.Error(fs.Name(), "OpenFile (O_APPEND) failed:", err)
+			continue
+		}
+		io.WriteString(f, "|append")
+		f.Close()
+
+		f, err = fs.OpenFile(path, os.O_RDONLY, 0600)
+		contents, _ := ioutil.ReadAll(f)
+		expectedContents := "initial|append"
+		if string(contents) != expectedContents {
+			t.Errorf("%v: appending, expected '%v', got: '%v'", fs.Name(), expectedContents, string(contents))
+		}
 		f.Close()
 	}
 }

memmap.go

diff --git a/memmap.go b/memmap.go
index 7053fa3..686d0e6 100644
--- a/memmap.go
+++ b/memmap.go
@@ -209,7 +209,17 @@
 	if os.IsNotExist(err) && (flag&os.O_CREATE > 0) {
 		file, err = m.Create(name)
 	}
-	return file, err
+	if err != nil {
+		return nil, err
+	}
+	if flag&os.O_APPEND > 0 {
+		_, err = file.Seek(0, os.SEEK_END)
+		if err != nil {
+			file.Close()
+			return nil, err
+		}
+	}
+	return file, nil
 }
 
 func (m *MemMapFs) Remove(name string) error {