Allow roxygen comments in object to be documented

[krlmlr]

useful e.g. to document implementation details inline

Example:

func <- function(a) {
  #' @param a an integer
  stopifnot(is.integer(a))

  #' @details obtw: this is how it works internally...
}

[hadley]

Hmmmmmmmmm. On one hand, I like the idea for RC/R6 and the implementation is simple. OTOH, I'd prefer to enforce a consistent style of roxygen usage. Let me sit on it for a bit.

[krlmlr]

As discussed: Added tests, no off-by-one errors found.