[ESS] recognizing R-alpha with ESS

[Stephen Eglen]

 > I'd assume they are considerably more rarely in use,
 > for everyone, so I'd be a bit reluctant to add more strings
 > there.  
 > Or is the above "slower" not noticable anyway?

ok - let's keep the default as it is, but I think it makes sense to
tell people to edit the ess-r-versions variable if they are going to
call other versions of R.

I'm not sure speed is a serious issue, but perhaps we could
test... (and if it is slow, perhaps "R-1" might be a candidate to be
deleted from ess-r-versions -- anyone using R-1.x.y?)

Stephen