Allow IPlugins to be passed directly to the compiler

[mavnn]

While loading plugins dynamically makes sense in the most common Ink use cases, it is a bit of a pain if you're just referencing the compiler directly in a project.

I'm using the code in this PR (which should be fully backwards compatible for any existing code) to pass in instances of IPlugin directly to the compiler, and was wondering if it would be useful for others as well.