* ChanHandler.cs: Move ShortenPost to the ChanHandler, since the way
  it shortens is more applicable to IRC than general purpose.

* ChanTools.cs: Stop GetThreadOP throwing NotSupportedException,
  instead convey the inability to parse the URL with a
  FormatException.
* Threading.cs:
* UrlTitling.cs:
* UrlTitling.csproj: Move the threading aspect to its own file.

* ChanHandler.cs: Add a comment.
* DomainLists.cs: Extra argument checks.

* MeidoBot.cs: Allow plugins to reply via the message. Also make Meido
  disposable.

* MeidoCommon.cs: Allow plugins to reply via the message.