Adds support for github pull requests

I decide to go with

```ruby
Headquarters::Github.pull_requests(query: "is:open")
```

over

```ruby
Headquarters::Github.open_pull_requests
```

because I feel it gives the caller more flexibility and power in the search, even if that means it has to know about github's search queries API.
It works, not exactly in the same way, but the result is the same. I'll update it.