[ESS] knitr

[Rasmus]

The following are just my opinions: 

> Do we have to know what is between << and >>= in order to add knitr
> support in ESS?

It doesn't need to, but I think there are quite a few options for Knitr,
also determining stuff like graphs, so it could be helpful.  Also, newer
iterations of ESS ship with auto-complete support, which could serve as
a nice interface.  How to do it smartly, I am not sure.  Ideally, I
guess it should just pick up whatever tags are supported 'live' (from
the 'Knitr bin' on the machine).

> As the first step of the implementation, ESS may just call
> library(knitr);knit(foo.Rnw) just like R CMD Sweave foo.Rnw.

Perhaps it would be better to just keep the generic `Sweave' + friends
buttons and make a customization option to choose either Knitr, Sweave
or something else.

-- 
And let me remind you also that moderation in the pursuit of justice
is no virtue