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.
isnt' this useless?
if the query is `nil` everything will work out the same way