Replace extract_figures.py with Javacript

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

Document Maximum Line Length

Close #134

Add information about where to store figures

Close #129

Add Style Guide for keyboard key

Thanks to @fmichonneau, @clintonroy and @willingc.

add keyboard key style

Fix concept-maps link

Close #176

Fix github-importer link

Close #177

"Offline" Capable

Add "Offline" Capable

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

Spelling

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.

Change recommendation for link

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

Style tags need to go on a newline

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>

Output episode figures in correct order