+ captions for each modal

[7yl4r]

captions for each silhouette/modal-popup should be added. Look at parrotfish as an example.

• Caption is in a markdown file in ./prep/plot_caption/.
• relative path to markdown file set in svg_elements.csv's modal_text column.

[7yl4r]

bah... trying to assign @mceachronfwc too, but github won't let me until the collaborator request is approved.