using dub to compile plugins

[Codifies]

I am currently using this dub.sdl

name        "runz80"
targetType  "executable"
lflags      "libz80/libz80.a"

however I will be creating a number of plugins, each plugin will 
consist of a single source file, I'd like the plugin source 
directory to be separate from main source directory and compile 
the plugins (.so) to a (binary) plugins directory

(the plugins will be dynamically loaded at runtime - I've 
previously done this in C so I don't anticipate any particular 
issues - famous last words!)

I could do this with a few simple rules in a Makefile, but I have 
no clue how to achieve this using dub.

can someone show me a concrete example of doing this ? Ideally 
just dropping a new source file into the plugins source folder 
should produce a new .so the next time dub is run, without having 
to explicitly add each plugin to the dub file...