Add support for remote config file

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, then remove your local `scalastyle-config.xml` file. Now you don't have to keep a copy of that file in every project!

If no local `scalastyle-config.xml` file is found, then the URL will be used. The contents of the URL are downloaded to a temporary file. 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).