Add `Discord` bot

[Ayumi]

Discord Bot

Discord is a proprietary freeware instant messaging and VoIP application and digital distribution platform designed for creating communities ranging from gamers to education and businesses. Discord specializes in text, image, video and audio communication between users in a chat channel.

Link: https://discord.com/

When running some tests on their apps, the app asks the user of they want to open external links through other browsers installed on the device. Therefore only their crawling bot seems to need to be added.

Example UA's:

Mozilla/5.0 (compatible; Discordbot/2.0;   +https://discordapp.com)
--
Mozilla/5.0 (compatible; Discordbot/1.0;   +https://discordapp.com)
Mozilla/5.0 (compatible; Discordbot/2.0;   +https://discordapp.com)
Mozilla/5.0 (compatible; Discordbot/2.0;   https://discordapp.com)
Discordbot/2.0ompatible; Discordbot/2.0;   +https://discordapp.com)
Discordbot/2.0
DiscordBot (https://github.com/Rapptz/discord.py   1.2.3) Python/3.6 aiohttp/3.5.4

Expected Results:

Browser Name: Discord
Browser Version: 2.0

PR see here: #500