Determine when to apply autoscroll

When shrinking the window, scrolling was limited to only the current
view buffer. This should implement auto-scrolling. It's a little bit
janky, but should be sufficient for now.

cui/scroll.go

@@ -0,0 +1,41 @@
+package cui
+
+import (
+	"strings"
+
+	"github.com/jroimartin/gocui"
+)
+
+func scroll(g *gocui.Gui, v *gocui.View, direction int) error {
+	if v != nil {
+
+		_, y := v.Size()
+		ox, oy := v.Origin()
+		cx, cy := v.Cursor()
+
+		if err := v.SetCursor(cx, cy-1); err != nil && oy > 0 {
+			if err := v.SetOrigin(ox, oy-1); err != nil {
+				return err
+			}
+		}
+		// If we're nearing a boundary
+		if oy+direction > strings.Count(v.ViewBuffer(), "\n")+y-direction {
+			v.Autoscroll = true
+		} else {
+			v.Autoscroll = false
+			v.SetOrigin(ox, oy+direction)
+			return nil
+
+		}
+
+	}
+	return nil
+}
+
+func cursorDown(g *gocui.Gui, v *gocui.View) error {
+	return scroll(g, v, 1)
+}
+
+func cursorUp(g *gocui.Gui, v *gocui.View) error {
+	return scroll(g, v, -1)
+}