Feat: expose date localizer (#1347)

I use date-fns and have custom localizer. Though I need to import
internals to create it. Would be good to have it in public api.

Also did you consider to use named exports instead of Object.assign?

src/index.js

@@ -1,12 +1,14 @@
 import Calendar from './Calendar'
 import EventWrapper from './EventWrapper'
 import BackgroundWrapper from './BackgroundWrapper'
+import { DateLocalizer } from './localizer'
 import momentLocalizer from './localizers/moment'
 import globalizeLocalizer from './localizers/globalize'
 import move from './utils/move'
 import { views, navigate } from './utils/constants'
 
 Object.assign(Calendar, {
+  DateLocalizer,
   globalizeLocalizer,
   momentLocalizer,
   Views: views,