CLI Hooks ionic:serve:before is blocking ionic serve

**Description:**

Defining an `ionic:watch:before` hook with a blocking script (e.g. an express server startup) will block `ionic serve`. Since the close resemblance to `npm hook scripts` it is expected to execute in a similar way; where `prestart` is non blocking for `start`.

**Steps to Reproduce:**

Clone repo: https://github.com/dnmd/cli-hooks

#### First

```bash
$ npm install
```
---

#### Non-blocking behaviour

```bash
$ npm start
```

This will executed both the `prestart` and `start` script.

---

#### Blocking behaviour

```bash
$ ionic serve
```

This will **only** execute the `ionic:watch:before` script.

#### Behaviour

When running `ionic serve`, the `ionic:watch:before` script is executed and opens a new terminal window, but **doesn't continue** the execution of `ionic serve`. When running `npm start`, the `prestart` script is executed and opens a new terminal window and **does continue** the execution of `start`.

**My `ionic info`:**
cli packages:

@ionic/cli-utils : 1.8.1
ionic (Ionic CLI) : 3.8.1

global packages:

Cordova CLI : 7.0.1

local packages:

@ionic/app-scripts : 2.1.3
Cordova Platforms : android 6.2.3
Ionic Framework : ionic-angular 3.6.0

System:

Node : v6.6.0
npm : 3.10.3
OS : Windows 10