Support GitHub enterprise (#73)

* Support GitHub enterprise

Add support for private GHE installations by enabling a new `--api-url` argument ... example usage:  `--api-url=https://MY_ENTERPRISE_DOMAIN/api/v3`.

* Add api-url to options documentation

Author: David Parker

docs/options.md

@@ -16,6 +16,7 @@ To pass it to the `GithubReleaseNotes` class, in the [configuration file](#confi
 
 | Command | Options | Description | Default |
 | ------- | ------- | ----------- | ------- |
+| `api-url` | **Optional** | Override the GitHub API URL, allows **gren** to connect to a private [GHE](https://enterprise.github.com/) installation. _e.g. `https://my-enterprise-domain.com/api/v3`_ | `null` |
 | `username` | **Required** | The username of the repo _e.g. `github-tools`_ | `null` |
 | `repo` | **Required** | The repository name _e.g. `github-release-notes`_ | `null` |
 | `action`| `release` `changelog` | The **gren** action to run. _(see details below for changelog generator)_ | `release` |

src/gren.js

@@ -861,7 +861,7 @@ GithubReleaseNotes.prototype.init = function() {
 
             var githubApi = new Github({
                 token: gren.options.token
-            });
+            }, gren.options.apiUrl);
 
             gren.repo = githubApi.getRepo(gren.options.username, gren.options.repo);
             gren.issues = githubApi.getIssues(gren.options.username, gren.options.repo);