Update a11y, support multiline captions

[Blackbaud-SteveBrush]

• Reworked image HTML to use figure and figcaption elements.
• Add support for multiline captions (originally, long captions would extend beyond the width of the image; this change restricts the caption's width to the width of the image).

[codecov[bot]]

Codecov Report

Merging #13 into master will not change coverage. The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #13   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           9      9           
  Lines          84     84           
  Branches        6      6           
=====================================
  Hits           84     84

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 206cfbe...473e720. Read the comment docs.