Allow adding other lessons

[ethanwhite]

Users should be able to do something like:

offlinedatasci add-lesson https://example.com

This structure follows the API cleanup in #92

This should work since we're basically just cloning websites, but we'll need to check some examples and figure out which of our current wget command sets (we have one for SWC and one for DC/LC) to use. Hopefully the SWC set will work for everything since it handles more edge cases.

[hturner]

If it is helpful to have something to play with, feel free to try https://warwick-stats-resources.github.io/r-foundations/. This is a complete course website, with pages for different sessions.

If that is too much (I'm not sure what a "lesson" looks like at the moment) you could pare down to the landing page for a specific session, e.g. https://warwick-stats-resources.github.io/r-foundations/content/01-intro/ or even just the slides for a specific session, e.g. https://warwick-stats-resources.github.io/r-foundations/slides/01-intro/.