Add support for retrieving tags from GitLab repos

Implement and document a new utility method for getting tags from
projects hosted on GitLab. This method supports GitLab.com as well as
projects hosted on independent servers.

docs/scraper-reference.md

@@ -236,3 +236,10 @@ To make life easier, there are a few utility methods that you can use in `get_la
     Returns the date of the most recent commit in the default branch of the given repository.
 
     Example: [lib/docs/scrapers/reactivex.rb](../lib/docs/scrapers/reactivex.rb)
+
+### GitLab methods
+* `get_gitlab_tags(hostname, group, project, opts)`
+
+  Returns the list of tags on the given repository ([format](https://docs.gitlab.com/ee/api/tags.html)).
+
+  Example: [lib/docs/scrapers/gtk.rb](../lib/docs/scrapers/gtk.rb)

lib/docs/core/doc.rb

@@ -270,5 +270,9 @@ module Docs
       timestamp = commits[0]['commit']['author']['date']
       Date.iso8601(timestamp).to_time.to_i
     end
+
+    def get_gitlab_tags(hostname, group, project, opts)
+      fetch_json("https://#{hostname}/api/v4/projects/#{group}%2F#{project}/repository/tags", opts)
+    end
   end
 end