Add .params argument to gh()

[jennybc]

I recently needed to call gh::gh() with params pre-collected in a list, i.e. the ones you would normally provide as name = value via .... In this case, you have to use do.call() or rlang::exec(). It would be interesting for us to add an explicit .params argument for this. Or we could import rlang and turn ... into dynamic dots.