Documentation examples of SSL config options; closes #116

I like the first three functions.  But I think that last one (ie, `option_help()`) creates more brittleness than it's worth.  And it's not too hard for `httr` users to find the correct page once their on the site.  And the cURL developers could change the web addresses so it's more complicated to find (eg, adding some hierarchy to the directories).  Apparently not all the options have pages currently, even when I Google them (eg, `CURLOPT_CLOSEPOLICY` and `CURLOPT_WRITEINFO`).

```r
for( curl_option in curl_options() ) {
  url <- build_help_url(curl_option)
  testthat::expect_true(
    url_ok(url), 
    info = paste("Option: ", curl_option), 
    label = "At least one cURL configuration option did not have a help page at the expected location."
  )
}
```

This test requires a minor refactor:
```r
build_help_url <- 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")
}

option_help <- function(x) {
  url <- build_help_url(x)
  BROWSE(url)
}
```