feat(task-alias): add 'alias' config

Allow defining task aliases through an `alias` config. `alias` is an object, with the key being the name of the alias to define, and the value is either the task name to alias (string), or array of task name strings.

Example:

```js
// warlock.json
"globs": {
   ...
}
"alias": {
   "b": "webapp-build",
   "bc": ["webapp-build", "webapp-compile"]
}
```