Oh man, I wish this existed! Unfortunately it doesn't. I've been using Microsoft's Portable Class Library (PCL), which uses a trimmed down version of .NET but does the job quite well if you're willing to give up some of the newer .NET features. MonoGame has a PCL version you can add as a reference via NuGet.
Unfortunately, Microsoft deprecated PCL a while back... September, maybe? You can still use it but I'm not sure if there are any detriments to this anymore. They came up with another approach that I can't remember the name of, but I haven't explored it because I didn't want to recreate all of my library projects so I just pushed on.
I'm not sure if there's a better way... I'm kind of interested to see if anybody posts any thoughts in this thread