wrap with a given format

[mgaitan]

add an option --wrap-as (-w) that wraps the content as a "code block" in a markdown file . For instance -w python will produce a file with ".md" extension and the content

 ```python
 <real content here>
 `` `

I guess this than force the extension for a given snippet, and with #6 we could have several snippets in the same document.