How to have common libraries/symbols?

I am working on multiple HTML5 projects, and they have some common symbols. Is there a way to have a common library of symbols such that if I edit a symbol in one place, it would reflect in all the projects? 

I tried saving them as custom assets, and although I am able to use the symbols in a different project, I am not able to edit them the way that I want, i.e. if I edit them in one project, the changes are only limited to that project.