* Make sure that when a connection stalls for too long (reaches 0 KB/s)
it will be restarted - otherwise sometimes we get 0 KB/s total download speed
for too long.

* Make sure one can specify more than one URL to download (and assign
finished segments to the new URLs).

* Implement FTP URLs.

DONE:
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* Make sure that upon receiving a Ctrl+C signal, the program will write
a continuation spec to a file, which will allow it to resume.