Allow to specify language for transform.HighlightCodeBlock

[skrysmanski]

As discussed here, it would be useful if one could overwrite the language in transform.HighlightCodeBlock - similar how it is possible to do so in transform.Highlight. At the moment transform.HighlightCodeBlock uses .Type which can't be changed.

One suggestion is to allow to overwrite the language via the options. (Although this could create some "interesting" scenarios if this option is then used in transform.Highlight or in a code block attribute in Markdown.)