allow plugins

[therealadityashankar]

one frustrating thing while I was writing python course content on my website was that I had to keep switching between wc-markdown and wc-code,

• I thought a cool thing would be if we could directly make the code blocks inside wc-markdown runnable by replacing them with wc-code instances, and if I could do this via a plugin
• this would also allow me to add auto language theme loading for wc-markdown