Support secrets API

[timholy]

Closes #159

No tests because doing anything with secrets requires authentication. But I used it to successfully create a DOCUMENTER_KEY secret for any of my personal repos that lacked it.

[KristofferC]

Not sure if the other exported functions like secrets and delete_secret should be in the README.

[timholy]

I could add them. Right now it seems a bit hit-or-miss:

julia> rdme = read("README.md", String);

julia> let yes = no = 0
           for name in names(GitHub)
               if occursin(string(name), rdme)
                   yes += 1
               else
                   no += 1
               end
           end
           println("yes: ", yes)
           println("no: ", no)
       end
yes: 118
no: 16

But since the large majority appear, I'll add it and then merge this.