Added batching behavior for page building.

Quite often file watcher gets many changes and each change triggered a
build. One build per second should be sufficient. Also added tracking for
new folders.

watcher/batcher.go

@@ -0,0 +1,56 @@
+package watcher
+
+import (
+	"github.com/howeyc/fsnotify"
+	"time"
+)
+
+type Batcher struct {
+	*fsnotify.Watcher
+	interval time.Duration
+	done     chan struct{}
+
+	Event chan []*fsnotify.FileEvent // Events are returned on this channel
+}
+
+func New(interval time.Duration) (*Batcher, error) {
+	watcher, err := fsnotify.NewWatcher()
+
+	batcher := &Batcher{}
+	batcher.Watcher = watcher
+	batcher.interval = interval
+	batcher.done = make(chan struct{}, 1)
+	batcher.Event = make(chan []*fsnotify.FileEvent, 1)
+
+	if err == nil {
+		go batcher.run()
+	}
+
+	return batcher, err
+}
+
+func (b *Batcher) run() {
+	tick := time.Tick(b.interval)
+	evs := make([]*fsnotify.FileEvent, 0)
+OuterLoop:
+	for {
+		select {
+		case ev := <-b.Watcher.Event:
+			evs = append(evs, ev)
+		case <-tick:
+			if len(evs) == 0 {
+				continue
+			}
+			b.Event <- evs
+			evs = make([]*fsnotify.FileEvent, 0)
+		case <-b.done:
+			break OuterLoop
+		}
+	}
+	close(b.done)
+}
+
+func (b *Batcher) Close() {
+	b.done <- struct{}{}
+	b.Watcher.Close()
+}