github: add issue and pull request templates

Fix: #5246
PR-URL: https://github.com/nodejs/node/pull/5291


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Steven R. Loomis <srloomis@us.ibm.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>