Allow installing the latest GitHub release

by passing `release = TRUE` to, or using the syntax `"user/repo@*` in `install_github`

Fixes #350.
I wonder if it's worth refactoring `with_mock()` so that you could do:

```R
with_mock({ 
  ...
}, github_resolve_ref.github_release = function(x, param) { 
  param$ref <- "latest-release"
  param
})
```