Documentation examples of SSL config options; closes #116

This seems moderately useful:

```R
httr_options <- function() {
  sort(RCurl::listCurlOptions())
}

curl_options <- function() {
  translate_curl(httr_options())
}

translate_curl <- function(x) {
  paste0("CURLOPT_", gsub(".", "_", toupper(x), fixed = TRUE))
}

option_help <- function(x) {
  stopifnot(is.character(x), length(x) == 1)

  if (x %in% httr_options()) {
    x <- translate_curl(x)
  }
  if (!(x %in% curl_options())) {
    stop(x, " is not a known curl option", call. = FALSE)
  }

  url <- paste0("http://curl.haxx.se/libcurl/c/", x, ".html")
  BROWSE(url)
}
```