allow specific repository to be encoded in remotes format

[kevinushey]

E.g. something like:

remotes::install_cran("[email protected]")

to say "I want to install devtools 2.1.0, as available from the repository called CRAN". The repository URL would be taken by matching the name against getOption("repos").

(It may also be useful to store both the repository name + URL as remote fields after install)

[jimhester]

We sort of have this idea in pak, there is cran::[email protected], though on cran:: is supported at the moment.

[jimhester]

I realize what you propose is an additional constraint on top of this, which I think makes sense.