Add rake task to create new release post

Mostly for @mattr- and @mojombo: a new rake task that auto-generates new release
posts based on the version given. Might consider adding the Liquid I wrote to
auto-generate Markdown links to various issues:

```ruby
{% assign issue_numbers = "1046|1204|1302|1198|1171|1118|1098|1215|1253|1271" |
split: "|" %}
{% for issue in issue_numbers %}
[#{{ issue }}]: https://github.com/mojombo/jekyll/issues/{{ issue }}
{% endfor %}
```