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/settings.go

@@ -213,6 +213,7 @@ func DefaultGlobalSettings() map[string]interface{} {
 		"savecursor":     false,
 		"savehistory":    true,
 		"saveundo":       false,
+		"scrollbar":      false,
 		"scrollmargin":   float64(3),
 		"scrollspeed":    float64(2),
 		"softwrap":       false,
@@ -248,6 +249,7 @@ func DefaultLocalSettings() map[string]interface{} {
 		"ruler":          true,
 		"savecursor":     false,
 		"saveundo":       false,
+		"scrollbar":      false,
 		"scrollmargin":   float64(3),
 		"scrollspeed":    float64(2),
 		"softwrap":       false,