Only try building R files if there are some

use anonymous function to generate markdown files

1.  Modify `read_markdown` to return list of (index, line, line_length).
2.  Modify `lesson_check.py` to make line-length checking optional.

Fix Rmd formatting

add tools to deal with episodes written in Rmd

1.  Renaming Ruby file to use underscore in name (consistent with other `bin` scripts).
2.  Generating `_includes/all_figures.html` for the first time when creating a new lesson.

fix #26: updated bundled deck.js, including the fix on browsercast

this bundle is used also for browsecast-less presentations
and thus browsercast should not unbind the space in this case
and that's solved now

consolidate badge classes to swc.css

Figures should be centered
and not overflow the screen.

the class removed are unused as we take advantage of classes provided by
Bootstrap instead.

Add documentation about card class

Better style for challenges