example shortcode: see if we can use a regex

https://github.com/twbs/bootstrap/blob/919b526f17a08cefba51c42b831c922adecb4a9f/site/layouts/shortcodes/example.html#L35-L38

The above basically extracts the class, removes bd-placeholder-img and bd-placeholder-img-lg.

I wonder if we can use findRE along with replaceRE to achieve the same thing.

Not a huge deal, it just feels like a more elegant solution.

Similarly we could try doing the same for:

https://github.com/twbs/bootstrap/blob/919b526f17a08cefba51c42b831c922adecb4a9f/site/layouts/shortcodes/example.html#L27-L29

This just extracts the <svg></svg>.