Add with_parent option to #repositories

## Overview
[Repositories#List-your-repositories](https://developer.github.com/v3/repos/#list-your-repositories) and [Repositories#List-user-repositories](https://developer.github.com/v3/repos/#list-user-repositories) returns response without parent and source repositories.
They are available in the response of [Repositories#Get](https://developer.github.com/v3/repos/#get).

So I added `:with_parent` option for `Octokit::Client::Repositories#repositories` to load parent repos together.

## Example
For now, you can load parent repos for `#repositories` by:
```ruby
repos = client.repositories("sferik")
forked_repo = repos.find(&:fork)
forked_repo.parent # => nil
forked_repo = client.repository(forked_repo.full_name)
forked_repo.parent # => {:id=>, ...}
```

After applying this pull request, you can do:
```ruby
repos = client.repositories("sferik", with_parent: true)
forked_repo = repos.find(&:fork)
forked_repo.parent # => {:id=>, ...}
```

You can load parent repository with cleaner code.