Add support for remote config file (downloads every time)

Add a new key: `org.scalastyle.sbt.PluginKeys.scalastyleConfigUrl`. It is an `Option[String]`. Default `None`.

To utilize this feature, set `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`.)

This feature is not enabled by default (because `scalastyleConfigUrl := None`), so old behavior is preserved.

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I have [another PR](https://github.com/scalastyle/scalastyle-sbt-plugin/pull/25) up which pulls the remote file once per some specified number of hours instead. That way if you don't have a network connection you'll still keep a copy of the remote file to work with, but you'll probably want to add `scalastyle-config.xml` to your `.gitignore`. I'm leaving both PRs up for consideration.