run an extra HEAD request on the expanded URL to determine
validity. This is an expensive operation, so recommended usage is to run
this only on URLs that did not seem to expand.
warn
show any warnings (API or otherwise) as messages
.progress
display a progress bar (generally only useful in
interactive sesions)
Value
data_frame (compatible with data.frame) of results
with the orignial URLs in orig_url and expanded URLs in
expanded_url)