Add downloading of data required for lessons

[ethanwhite]

Problem

Need to include all data files used in lessons. For data that is hosted on the same domain this already working, but some of the data used in lessons is linked to external sources (this is actually good practice when using someone elses data). Rewriting the urls is probably too difficult and fiddly to do without downloading the entire internet when data is externally linked (but we could try).

Suggested approach

Add new install datasets functionality where we:

1. Store a list of datasets to install by default
2. Allow the user to add additional datasets like they can add additional packages (by passing a url)
3. Puts the datasets in a data subdirectory in the lessons directory and then include a datasets section on the lessons index.html page.

[ethanwhite]

There was a follow up question about this at the Carpentries Offline meeting this week.

@garezana - if you've made some progress it might be useful to push up a branch (even if the work isn't complete yet) so that others can see where things are and help out if useful