Enhanced plugin support and tooling

[ztreinhart]

#28 Adds support for loading a binary plugin into VHS at runtime. This plugin can add or replace any number of sources, sinks, formats, or modifiers.

Looking forward, it would be useful to support loading multiple plugins, as well as metadata and versioning information for plugins to ensure compatibility. We may eventually want to support signing plugins as well. Implementing these features and future plugin features will require the development of our own tooling around the standard plugin package.

It's important that this plugin tooling provide good user experiences for both developers wanting to build plugins and non-developer end users who simply want to utilize one or more plugins that others have developed.

@andrewhare, perhaps you could summarize your proposed approach to this tooling?