Do not return invisible(pr) for pr_*()

[schloerke]

I had done some magrittr style work and did not immediately run the router. When the commands finished, the router did not display... that was confusing. Now it will display.

Without PR:

> pr() %>% pr_get("/route", identity)

With PR:

> pr() %>% pr_get("/route", identity)
# Plumber router with 1 endpoint, 4 filters, and 0 sub-routers.
# Call run() on this object to start the API.
├──[queryString]
├──[body]
├──[cookieParser]
├──[sharedSecret]
└──/route (GET)

PR task list:

• Update NEWS
• [na] Add tests
• Update documentation with devtools::document()