wget -c equivalent. If told to resume we should be able to read the length of the current file and use the HTTP Range header to resume the download. In the case where Range is not supported by the server, we would revert to restarting the download and output a warning.

Steps to reproduce

iwr http://someurl -outfile bigfile.zip
// assume download gets stopped prematurely

Expected behavior

iwr http://someurl -outfile bigfile.zip -resume

Actual behavior

Need to restart the download from the beginning