Add file code block

[d0c-s4vage]

Describe the Feature Request A file codeblock should be added that can load source from external files

Example Markdown

```file
path: path/to/file
relative: true # default is relative to the slide source
lang: pygments language
transform: null # a command to run in the user's shell prior to rendering the loaded source
                # this should also be prior to calculating lines to display
lines:
  start: 0 # default
  end: null # default
```

Additional notes

This will make presenting source code, binaries, etc much easier by pulling in sources from external files. I've run into this many times already since starting to use lookatme for teaching.