Add Plumber$flags

[schloerke]

Fixes #592

• Related: avoid print mount recursion fix #592 #683
  Fixes @plumber tag should verify that the returned router matches the supplied router #707
• Related: Checking if router modified in plumb block is the one the argument #710

Now throws an error:

#* @plumber
function(pr) {
  pr() %>%
    pr_get()
}

Now does not print forever:

a <- plumber$new()
b <- plumber$new()

a$mount("B", b)
b$mount("A", a)

print(a)

PR task list:

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