Add additional query parameters to GetRepositoryIssues #56
Description
`GetRepositoryIssues` currently supports `labels` and `state` query parameters. The GitHub API supports several more that would be useful:
| Parameter | Type | Description |
|---|---|---|
| `sort` | String | `created`, `updated`, or `comments`. Default: `created` |
| `direction` | String | `asc` or `desc`. Default: `desc` |
| `since` | String | ISO 8601 timestamp. Only issues updated at or after this time |
| `per_page` | Integer | Results per page (1-100). Default: 30 |
These parameters can be added to the existing `GetRepositoryIssues.apply()` method and `Repository.get_issues()` as additional defaulted parameters, and to `_build_url()` for URL construction.
See also Discussion #54 for whether `sort` and `direction` should use union types instead of strings.