Add init and finish hook for knitr::knit()

[cderv]

This idea came up with discussion for #2219 in r-lib/rlang#1551

At first it was something that could have been needed as rlang needed to set thinks at start of knitting, and reset after knitting. Solution was done another way (r-lib/rlang@6b3cbe9)

However, the idea is interesting to have those hooks available for other to extend knitr. So leaving this issue open to keep track of the idea, but we can wait for another real use case.

[yihui]

I'm okay with adding these hooks.

knitr/R/output.R

Line 254 in a007b37

res = process_file(text, output)