github: implement .gh-repo command, to set/view channel's linked repo

Considering a special "clear" argument, but that's for later.

sopel_modules/github/github.py

@@ -175,6 +175,24 @@ def issue_info(bot, trigger, match=None):
     bot.say(''.join(response))
 
 
+@commands('gh-repo')
+@example('.gh-repo sopel-irc/sopel-github')
+@require_chanmsg('[GitHub] You can only link a repository to a channel.')
+def manage_channel_repo(bot, trigger):
+    """
+    Set the repository to use for looking up standalone issue/PR references.
+    """
+    allowed = bot.channels[trigger.sender].privileges.get(trigger.nick, 0) >= OP
+    if not allowed and not trigger.admin:
+        return bot.say('You must be a channel operator to use this command!')
+
+    if not trigger.group(2):
+        return bot.say(bot.db.get_channel_value('github_issue_repo', trigger.sender))
+
+    bot.db.set_channel_value('github_issue_repo', trigger.sender, trigger.group(3))
+    bot.reply('Set linked repo for %s to %s.' % (trigger.sender, trigger.group(3)))
+
+
 @url(commitURL)
 def commit_info(bot, trigger, match=None):
     match = match or trigger