Add R package ignores to vendor.yml

Ignore vignette and external data directories which contain no R source code
Change R package ignores to absolute paths
Merge branch 'master' into 1042

Conflicts:
	lib/linguist/vendor.yml
Merge commit 'refs/pull/1042/head' of github.com:github/linguist into 1042

Conflicts:
	lib/linguist/vendor.yml
Merge pull request #1256 from github/1042-update

1042 update