irccloud formatting helper 2

Add Markdown parser and preview rendering in the input box. There is a button right beside the Emoji Selector that will display either "M" (markdown mode) or "T" (plaintext mode). Click to change mode.
In markdown mode, use formatting such as *italic* _italic_ **bold** __underline__ ~~strikethough~~ in the input box. A preview will display below the input box that shows how the input will be interpreted.
In plaintext mode, it will behave just like regular IRCCloud.

Adapted from: https://greasyfork.org/en/scripts/17615-irccloud-formatting-helper and https://github.com/dogancelik/irccloud-sws/