Set up registration bot on Github like Registrator.jl

[mtfishman]

It would be nice if we could register new package versions by commenting on commits on Github, rather than executing commands at the Julia REPL. It would require setting up registration bot on Github like Registrator.jl, though that is only designed for the General registry right now: JuliaRegistries/Registrator.jl#39. (Brought up by @lkdvos at some point.)