fix: support point-in-time events in the Agenda view (jquense#1246)

Some events may have the same date/time, and are meant to represent
point-in-time events, such as deadlines. These are not all day
events, but they also should not be represnted as
`${startTime} - ${endTime}` since the values will be the same.

This will cause an event that is not marked as all day, and where
the start and end are equal, to show as:

```js
// old
`${startTime} - ${endTime}`

// new
`${startTime}`
```

examples/events.js

@@ -1,3 +1,5 @@
+const now = new Date()
+
 export default [
   {
     id: 0,
@@ -103,4 +105,10 @@ export default [
     start: new Date(new Date().setHours(new Date().getHours() - 3)),
     end: new Date(new Date().setHours(new Date().getHours() + 3)),
   },
+  {
+    id: 15,
+    title: 'Point in Time Event',
+    start: now,
+    end: now,
+  },
 ]

src/Agenda.js

@@ -129,7 +129,9 @@ class Agenda extends React.Component {
     let start = accessors.start(event)
 
     if (!accessors.allDay(event)) {
-      if (dates.eq(start, end, 'day')) {
+      if (dates.eq(start, end)) {
+        label = localizer.format(start, 'agendaTimeFormat')
+      } else if (dates.eq(start, end, 'day')) {
         label = localizer.format({ start, end }, 'agendaTimeRangeFormat')
       } else if (dates.eq(day, start, 'day')) {
         label = localizer.format(start, 'agendaTimeFormat')