Pull Request carpentries/styles#330
Closes carpentries/styles#343

1. Fix the script so that all figures are displayed.
2. Add image numbers.
3. Don't add headers for episodes without images.
4. Add horizontal rulers to better separate adjacent images.
   Don't add a ruler after the last image in the episode.
5. Add empty lines to `figures.md` to improve code readability.
6. Use classes for styling text and figures

Pull Request: carpentries/styles#327

Close carpentries/styles#325

Close swcarpentry/styles#244

Close swcarpentry/styles#249

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

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>

Close #155

This is based on
https://github.com/swcarpentry/instructor-training/pull/437/files and
https://github.com/swcarpentry/instructor-training/pull/439/files,
both write by @ErinBecker.

Includes @drjwbaker's description of Library Carpentry.

Close https://github.com/swcarpentry/styles/issues/101.

Change text color, background color and padding around inline code blocks

Example of the problem: see "Ten Things You Need To Know" from http://swcarpentry.github.io/lesson-example/
For numbered lists that have more that 9 elements 1 em padding on the left is not sufficient. The result is that "1" in numbers "10" and above is printed on top of the border of encapsulating HTML block. 2 em padding solves this problem for lists with less that 100 elements. It should be safe to safe to assume that numbered lists in Carpentries' lessons should not have more than 99 items.