Implement: Search box autocomplete.

[luckydonald]

https://derpibooru.org/tags/autocomplete.json?term=<term>

// 20160307185305
// https://derpibooru.org/tags/autocomplete.json?term=luckyd

[
  {
    "label": "artist:luckydonald (4)",
    "value": "artist:luckydonald"
  },
  {
    "label": "artist:luckyde (1)",
    "value": "artist:luckyde"
  },
  {
    "label": "artist:luckydragoness (0)",
    "value": "artist:luckydragoness"
  }
]