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Android N will have native quick reply feature (called Direct Reply), and while WhatsApp will probably use it, it won't work for pre-N (although it has implemented its own quick reply from notification for all Android version).

However, there are some apps that can emulate the direct reply feature on Android N:

quickReply will show you separate notifications for every contact including reply action buttons, so you can easily write quick replies without opening the messaging app.

QUICKREPLY SUPPORTED APPS:
WhatsApp, Telegram, Threema, Skype, Line, WeChat, Kik, EvolveSms, YAATA, Textra, ChompSms, Hangouts, Handcent SMS...

Advanced heads-Up notifications for your phone! QuickReply lets you send messages directly from the notification! ...

MAIN FEATURES:
Heads-up Quick Replies (5.0+, limited on 4.3+, see below for supported apps)

QUICKREPLY REPORTED WORKING:
WhatsApp, Telegram, Threema, Skype, Line, WeChat, Kik, EvolveSms, YAATA, Textra, ChompSms, Hangouts, Handcent SMS...

Both apps are developed by Jawomo, so it's not really surprising they have similar feature and supported apps...