Hugo v0.146 removed access to embedded templates like "_internal/shortcodes/figure.html".
This commit adds the template as figure-default.html.
This fixes build errors reported and partially solved in issues #2052 and #2093
when using the figure shortcode with the "default=true" option.
Add the "target" parameter to the figure.html shortcode.
If defined then add this target property to the href of the figure,
which allows the link to, e.g., open in a new tab.
Add description of target to docs
