Handle external wget dependency for macOS and Windows

[ethanwhite]

Our approach to downloading lessons (and in the future other web content) relies on wget, because wget is amazing and solves all of our needs out of the box. However, while wget comes preinstalled on almost any Linux OS it is not necessary present on macOS and Windows.

macOS can be solved by recommending a conda-based install and implementing #55, because conda distributes wget for macOS. However, it doesn't currently distribute it for Windows.

For Windows we could either: 1) work with the folks maintaining the conda wget recipe to see if Windows support can be added; or 2) see if we can have offlinedatasci download and use a Windows executable from https://eternallybored.org/misc/wget/