[ESS] Manually turn on ess script mode

[Tyler Smith]

Hi,

I'm marking a bunch of student R assignments. Some of the students have
submitted files without the .R extension, so ess doesn't know to turn on
when I open them. I can resave them with the correct extension and
reopen them. However, I could skip this step if there was a way to
explicitly tell emacs to start ess[s] mode. Is this possible? The manual
only says that the mode should start automatically, but doesn't indicate
if it's possible to make it start when the hooks don't get triggered.

Thanks,

Tyler