Fix #54

[sdruskat]

Rendering of markdown inside an HTML block tag is disallowed per default (<figcaption> is per default rendered as a block by most browsers, cf. http://www.w3schools.com/tags/tag_figcaption.asp). In order to allow markdown to be rendered, kramdown has the attribute markdown which can be set to 1 to allow rendering. Hence, this commit adds a span with markdown='1' to the <figcaption> tag in main_column_img.rb. Cf. also http://stackoverflow.com/a/35276926/731040.