wikipedia: use bot.say()'s trailing parameter for ledes ("snippets")

We sacrifice the ability to know for sure if another Sopel bot's wiki
output was what triggered this call, but gain the ability to include
more of the article text (and test out a hack of the `trailing` param
that will be useful for other plugins with truncatable content that's
not at the very end of the message).

Replaced the `msg_url == trigger` check with a heuristic: Assume that
the triggering user was a Sopel bot if the trigger started with
`[wikipedia] ` and ended with ` | <our wiki page URL>`. Obviously it
won't be a super effective check if two different versions of Sopel with
different Wikipedia output meet, but the old check would also break in
that case. The change seems worthwhile.

sopel/modules/wikipedia.py

@@ -152,26 +152,33 @@ def mw_search(server, query, num):
 def say_snippet(bot, trigger, server, query, show_url=True):
     page_name = query.replace('_', ' ')
     query = quote(query.replace(' ', '_'))
+    url = 'https://{}/wiki/{}'.format(server, query)
+
+    # If the trigger looks like another instance of this plugin, assume it is
+    if trigger.startswith(PLUGIN_OUTPUT_PREFIX) and trigger.endswith(' | ' + url):
+        return
+
     try:
         snippet = mw_snippet(server, query)
     except KeyError:
         if show_url:
             bot.reply("Error fetching snippet for \"{}\".".format(page_name))
         return
-    msg = '{} | "{}"'.format(page_name, snippet)
-    msg_url = msg + ' | https://{}/wiki/{}'.format(server, query)
-    if msg_url == trigger:  # prevents triggering on another instance of Sopel
-        return
+
+    msg = '{} | "{}'.format(page_name, snippet)
+
+    trailing = '"'
     if show_url:
-        msg = msg_url
-    bot.say(msg)
+        trailing += ' | ' + url
+
+    bot.say(msg, trailing=' […]' + trailing)
 
 
 def mw_snippet(server, query):
     """Retrieves a snippet of the given page from the given MediaWiki server."""
     snippet_url = ('https://' + server + '/w/api.php?format=json'
                    '&action=query&prop=extracts&exintro&explaintext'
-                   '&exchars=300&redirects&titles=')
+                   '&exchars=500&redirects&titles=')
     snippet_url += query
     snippet = get(snippet_url).json()
     snippet = snippet['query']['pages']