Verify mirrors

[jrdnbradford]

From discussion in #82.

We need an unexported function to test whether or not a particular mirror is indeed a mirror and is working. Checking that it is in the mirrors list won't quite do the job as many mirrors aren't listed there.

This will prevent us from having to do workarounds such as that fixed in #84.

[jrdnbradford]

Since gutenbergr currently only supports http, https, and ftp mirrors, perhaps the check should be limited to these. We could do something like:

is_gutenbergr_compatible_mirror <- function(url) {
  base_url <- sub("/+$", "", url)
  readme_url <- paste0(base_url, "/README")
  readme <- read_url(readme_url)
  contains_pg_string <- any(grepl("GUTINDEX.ALL", readme))
  contains_pg_string
}

If read_url is able to read a root README and it has a reference to GUTINDEX.ALL then it's most likely a running mirror gutenbergr can handle. We can add this check to a few key places, like if try_gutenberg_download fails or when gutenberg_get_mirror is run and there isn't already the gutenberg_mirror option set.

There are obviously several conditions we can check for besides/instead of this. Let me know what you think.