Fix CSS to use .language-X class

Advise users not to edit .md files generated from .Rmd files

secure hyperlinks to pre-empt server-side HTTPS redirects

Fix bin/lesson_initialize.py

If _extras/figures.md exists,

    $ ./bin/lesson_initialize.py

will return

    Warning: _extras/figures.md already exists.
    **Exiting without creating files.**

Close #150

Close #152.

Syntax highlight

Based on https://github.com/swcarpentry/lesson-example/pull/144/files

Close https://github.com/swcarpentry/lesson-example/issues/145.

Close #186

Adopt email instead of contact as keyword

Close #178

Replace extract_figures.py with Javacript

Demo at https://github.com/swcarpentry/lesson-example/pull/132/.

add keyboard key style

Fix concept-maps link

Close #176

Fix github-importer link

Close #177

Add "Offline" Capable

This is based on https://github.com/swcarpentry/lesson-example/pull/127/.

Fix episode nav wrapping

By doing this, when a page is heavily zoomed in (e.g. 500%) the navbar
icons stay in the same row, rather than breaking onto two different rows.

Load methods package

Building `md` from `Rmd` in a script like this will break if the `methods` package is not called and the code includes class object routines. See https://github.com/r-spatial/sf/issues/478

SWC uses block inline attribute lists for e.g. code blocks, but I think this HTML tag is much simpler and more understandable than <pre>*Enter*{:.keyboard} ...</pre>