Support Go 1.8 plugins

[icecrime]

Description

Golang is adding support for plugins: we should leverage that! This would allow projects to add non-generic operations for their particular use case (which probably version-label and others in the current tree probably should be).

What should be done?

The tool should look for plugins in a subdirectory, and dynamically load any operations it can find. The plugins would likely provide the OperationDescriptor interface.

[icecrime]

Until Go 1.8 is mainstream, I'd love to have a Makefile rule that builds with a plugin build tag to support that feature.