Allow for quiet first-time loading using library()

[vjp23]

While working with an automated Docker build benefiting from the needs library, it took a lot of extra work to respond to the autoload question when loading needs.

I had to add a line in my Dockerfile to vim edit usr/local/lib/R/site-library/needs/extdata/promptUser to contain a 0 instead of a 1 before loading needs.

It would be wonderful if needs could use the quietly or quiet option in the library() function to assume Yes to the autoload question to avoid this problem when first loading needs in interactive sessions in automated builds.

Here's the Stack Overflow question for this issue: https://stackoverflow.com/questions/53547329/automatically-respond-yes-to-r-library-prompt/53547539#53547539