feat: add callback on codeblocks

[youkwhd]

Changed the writing behavior of codeblock transformer (buffering it first)

An example of a use case:

(markdown/md-to-html-string "```python\ndef f(x):\n    return x * 2\n```"
                       :codeblock-callback (fn
                                             [code language]
                                             (trim (clygments/highlight code language :html))))

Code above would syntax highlight codeblocks without being bothered to convert the Markdown to HTML first then calling libraries like highlight.js with JavaScript

References:

• https://en.wikipedia.org/wiki/Data_buffer
• https://github.com/bfontaine/clygments
• https://pygments.org/

[yogthos]

Thanks for the pr, just pushed out 1.11.5 with the feature to Clojars.