Overhaul container integration

[earldouglas]

This issue subsumes #407, #426, and #431.

There are two areas of concern about the way container integration currently works:

1. Embedded Containers

   The current architecture is built around embedded containers (jetty-runner for Jetty, webapp-runner for Tomcat) launched as forked JVM processes. This is fine for quick-and-dirty development and testing, but does not make it easy to develop in a real, locally-installed instance of a Servlet container.

2. Container Preference

   According to a recent survey, the vast plurality of folks are using some form of Tomcat, with JBoss/Wildfly in second place, Jetty in third, and others trailing by a mile. This plugin has historically prioritized embedded Jetty, with additional support for embedded Tomcat and user-configured custom embedded containers.

It might be worth re-focusing this plugin around real, locally-installed instances of Servlet containers, with additional support for a single embedded container. This would decouple container configuration from project configuration, and would provide first-class support for development in more realistic environments. The convenience of not needing a local container installation would still be there, but the standard use case would integrate with a user's own local container installation. This would also let us factor out support for the very deprecated jetty-runner.