Add support for remote config file (downloads once per day)

To utilize this feature, set `org.scalastyle.sbt.PluginKeys.scalastyleConfigUrl` in your `Build.scala` to be the URL at which your config file can be found. Now you will automatically download a new version of the config file remotely, once per day, so you keep up-to-date.

If no local `scalastyle-config.xml` file is found, or if the `lastModified()` timestamp is greater than a day ago, then the URL will be used. The contents of the URL are downloaded to the `scalastyle-config.xml`. If it is not a correct URL or the file at the URL isn't good XML you will get an error. (Common exceptions are `java.net.MalformedUrlException`, `java.io.FileNotFoundException`, and `org.xml.sax.SAXParseException`.)

By default you will use [this repo's config](https://raw.githubusercontent.com/scalastyle/scalastyle-sbt-plugin/master/scalastyle-config.xml).

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This is in contrast to [my other PR](https://github.com/scalastyle/scalastyle-sbt-plugin/pull/24) which downloads every time. If you are doing that and you don't have a network connection, you won't be able to run scalastyle. You'd have to remember to manually download the config. On the other hand, this version will force you to either commit the `scalastyle-config.xml` or add it to your `.gitignore`. I leave both options up as a choice.