install_github only once ever.

install_github2.r

library(git2r)
library(devtools)
library(drat)

install_github2 <- function(repo) {
	dest <- tempfile()
	dir.create(dest, recursive=TRUE)

	# download
	repo <- git2r::clone(paste0('https://github.com/',repo,'.git'),dest)
	on.exit(unlink(dest), add = TRUE)

	# install
	devtools::install_local(dest)

	# build
	buildpath <- tempfile()
	dir.create(buildpath, recursive=TRUE)
	on.exit(unlink(buildpath), add = TRUE)
	devtools::RCMD('build',c('--as-cran',dest),buildpath)

	# drat
	files <- dir(buildpath) 
	drat::insertPackage(file.path(buildpath,files[1]))
}

# try it:
#install_github2('cttobin/ggthemr')