Add a scroll bar option

The option is `scrollbar` and is off by default. The scroll bar is
not interactive (you can't click and drag it) but this will likely
be fixed in the future.

Ref #869

cmd/micro/scrollbar.go

@@ -0,0 +1,18 @@
+package main
+
+type ScrollBar struct {
+	view *View
+}
+
+func (sb *ScrollBar) Display() {
+	style := defStyle.Reverse(true)
+	screen.SetContent(sb.view.x+sb.view.Width-1, sb.view.y+sb.Pos(), ' ', nil, style)
+}
+
+func (sb *ScrollBar) Pos() int {
+	numlines := sb.view.Buf.NumLines
+	h := sb.view.Height
+	filepercent := float32(sb.view.Topline) / float32(numlines)
+
+	return int(filepercent * float32(h))
+}