I’m sure this is a very easy add (easier than the existing syntax highlighting request, which I’d also love to see added) and could be a temporary workaround to not being able to add Colored Blocks or Callouts as described here:
…Users would simply add code blocks and change the color to yellow or whatever. They could also add a emoji/icon to the beginning as shown in the ^screenshots, but then you’d of course have some ugly indenting if the text is long enough to wrap.
Obviously one should almost never release a feature to only serve as a temporary workaround, but I’m sure many users would continue to use this after Coda adds true support for colored blocks/callouts, as using a color other than light grey could be desired.
P.s. Please please PLEASE add support for colored blocks/callouts!! 
I recommend checking out Craft.do for inspiration — I discovered that app yesterday and they do a fantastic job with this.