[WIP] Create icon plugin

## What's it?
This pull-req is the implement of Icon Plugin.
I need **many many** Review fill with your KINDNESS!! :heart_eyes: :+1: :sparkles:

## How to implement
At first, I create the TEST and commit it with failures.
Then, I resolve the failures and commit again.

## ToDo
- [x] Add IconPluginTest.php
- [x] Add IconPlugin.php
- [x] Add unsolved tests
  1. call inline method of icon plugin
  2. call icon plugin with no params
  3. call icon plugin with params(ie. glyphicon, search)
- [x] Resolve icon plugin test of calling inline method
- [x] Resolve icon plugin test of calling with no params
- [ ] Resolve icon plugin test of calling with params(ie. glyphicon, search)

...and more!