PR merge messages need more info

[half-duplex]

Current message when a PR is merged:
<Sopel> [sopel] dgw approved pull request #1516 https://git.io/fjqGw
This should have at least the name of the user who opened it and the title, like normal PR link info display:
<Sopel> [GitHub] [sopel-irc/sopel #1539] deathbybandaid: Provide a simpler way to kick users from channels. | this adds a `bot.kick(text, channel, nick)` ability to the bot, instead of using `bot.write(['KICK', channel, nick], text)`. ...

It would be good to check other simple action messages, like issue/pr closed, for format consistency or similar missing info.

[dgw]

This idea also came from IRC discussions in Sopel's channel.

Also, I should have stuck with my gut for which milestone to assign this. I was right the first time. >.<

[dgw]

I want to get 0.2.0 out relatively soon, and this issue warrants spending enough time to consider all the different possible outputs and develop a unified structure to them all that will allow presenting more details in more places without seeming cluttered or funny-looking. Punted to 0.3.0. 😸