Add gen task to Middleman to allow scaffolding of monthly meetups post

## Description

At the request of @zeeshanlakhani I've added a quick and dirty task to Middleman's CLI to allow quick generation of the monthly meetups post.

### Example

```shell
$ bundle exec middleman gen meetups -t "Super cool meetups" -d 2014-12-01
```

Will generate a file named `2014-12-01-super-cool-meetups.html.markdown` in the `/source` directory that looks like so:

```html
---
title: Super cool meetups
date: 2014-12-01
author: Boatswain Miller
category: news
tags: meetup, chapters
label: Meetups
description: Papers We Love Meetup Schedule for 2014-12-01
---

We have another great line-up of meet-ups scheduled for DATE across a number of our chapters:

**CHAPTER MM/DD**: [TITLE](LINK)

---

<img class="left no-shadow" alt="SPONSOR NAME" style="width: 120px" src="/images/SPONSOR_IMG.png" />
The **CHAPTER** would like to give special thanks to [SPONSOR](SPONSOR_LINK) for sponsoring the ITEMS for the MONTH meetup.
```

You can then edit the text as needed to create a schedule of meetups for the month. The options for `gen` are:

Option | Alias | Description
---------|---------|-----------------
title | -t | The title of the scaffolded post
date | -d |The date of the scaffolded post (defaults to today's date if omitted)
author| -a | The author of the scaffolded post

```bash
$ bundle exec middleman gen meetups -t "Super cool meetups" -d 2014-12-01 -a "Zeeshan"
```