external file code block: initial implementation

[cart]

Adds the ability to embed external files as code blocks in markdown text. This opens up the doors to validating these files using other tooling (such as cargo build for rust).

The syntax is:

{{file_code_block(path="templates/base.html" lang="html")}}

More context in #143

[cart]

This could be improved in the future by adding the ability to provide "line number ranges" to filter down to a subsection of a file.