Add sbt scripted tests for plugin

[valencik]

To help automate testing of the sbt plugin, we should add some sbt scripted tests.

I think we should follow the Laika setup as much as possible.

• examples in the sbt-test dir
• setup in build.sbt

We don't need much to start, just a simple setup would be good.
We could have a single markdown file and just assert that the index exists after running the tlSite command.

Here's a blog post with an overview of the process:
https://blog.ssanj.net/posts/2017-09-07-how-to-setup-the-sbt-scripted-test-framework.html