Add IconSearch plugin

[sg-2512]

{
"name": "IconSearch",
"title": "IconSearch - 355,000+ Vector Icons",
"author": "IconSearch",
"description": "Search, style, and insert 355,000+ vector SVG icons directly into Sketch with 1-click device auth.",
"homepage": "https://iconsearch.info/sketch",
"appcast": "https://iconsearch.info/sketch/sketch-plugin.json"
}