This PR adds save to wayback functionality.

It can do a single url

r <- save_wayback("qdr.syr.edu")
r
 $url
 [1] "https://qdr.syr.edu/"

$archived_snapshots
$archived_snapshots$closest
$archived_snapshots$closest$status
[1] "200"

$archived_snapshots$closest$available
[1] TRUE

$archived_snapshots$closest$url
[1] "http://web.archive.org/web/20190111181645/https://qdr.syr.edu/"

$archived_snapshots$closest$timestamp
[1] "20190111181645"

or it can save a list of urls:

r <- archiv(c("qdr.syr.edu", "notaurl", "www.google.com"))

                    url status available?
1  https://qdr.syr.edu/    200       TRUE
2               notaurl    000      FALSE
3 http://www.google.com    200       TRUE
                                                      wayback_url
1  http://web.archive.org/web/20190111190523/https://qdr.syr.edu/
2                                                   url not found
3 http://web.archive.org/web/20190111190630/http://www.google.com
       timestamp
1 20190111190523
2        unknown
3 20190111190630