Allow specified R version in electrify() and install_r()

[gorcha]

Currently electrify() always installs the most recent version of R on Windows, and uses a hard coded version for Mac.

It'd be great if there were an option allowing the user to specify the R version in electrify() or install_r().

[bianchenhao]

I'm troubled by this too!

[chasemc]

I've added an argument that lets you specify the url to the mac tar.gz and will merge it in when it passes checks.

The versioning with Windows was meant to be achieved by using MRAN to keep R and package installation in sync by specifying a date, that will install packages available on that date, along with the R version that is most recent as of that date.
electrify(mran_date = "2019-08-05", cran_like_url = NULL)

[chasemc]

Feel free to reopen if for some reason the MRAN method isn't sufficient.