Convert load_urls to a Stream

[Glutexo]

The downloading should work as a stream, everything from picking the URL from the file through the download from the Internet to the local drive file write should happen separately for every item, not by repeated enumeration over a list. This required another version of the download/3 function accepting a Stream and returning a Stream. Started with converting the load_urls/1 function to a Stream.

[Glutexo]

I now think that the download/3 function that accepts a list of URLs is unnecessary. Traversing an enumerable is a trivial operation and we don’t need to abstract it and separately test it. Here, with the introduction of streams, it shows us that there are more ways to call the download/3 function with a single URL and until now, it was only limited to lists.

[Glutexo]

The aforementioned is addressed in #67.

[Glutexo]

Merged current elixir and resolved conflicts.

[Glutexo]

Merged current elixir and resolved conflicts. I did not want to duplicate the download_urls/3 function now when it contains the file name functionality. It stroke me as a bad piece of design. Thus only kept the load_urls/1 method conversion with no further changes.