Add event support for corral.json

[damon-kwok]

The Event allow Corral to trigger and run custom tasks as it processes commands.
The difference between it and Plugin is: the event only takes effect for the current project.

corral.json:

{
  "deps": [
    {
      "locator": "github.com/ponylang/valbytes.git",
      "version": "0.5.0"
    }
  ],
  "info": {}, 
  "event": [
    {
      "on_init": "echo 'corral init'",
      "on_build": "echo building",
      "on_clean": "echo clean",
      "on_fetch": "echo fetch",
      "on_update": "echo update",
    }
  ]
}

[SeanTAllen]

Why? You need to provide problems that exist and how this will solve it, otherwise its just features for the sake of features.