Newline at end of file linter

[jimhester]

while readLines does throw a warning in this case we should also generate a lint (and probably catch the warning).

This error is basically impossible to generate using vim and emacs, but very easy to do with RStudio.

[Arcanemagus]

This is also pretty easy to trigger within Atom since the new integration for that lints the contents on the fly.

[jimhester]

@Arcanemagus I just updated the atom linter code to append a newline to the file from the buffer, which fixes this issue AtomLinter/linter-lintr@7ff6d0c.

[MichaelChirico]

OG version of #675 / #719; closing as superseded by those