Currently, only one plugin is allowed per library. This matches the typical use case. However, there are use cases where multiple plugins per assembly is appropriate.

This corresponds with #7 wherein a plugin will be conditionally loaded based on a set of criteria.

There are two ways to do this:

1. Just allow multiple plugins per assembly overall - if the assembly has multiple, add all (that match the predicate) to the plugin tree. This may require some more organization.
2. Get a list of all plugins in the assembly, apply the filter, then assert that there is only a single plugin or else fail to load that assembly.

For the first impl, I would like to propose the 2nd option, but we can come back later and make it the first if need be.