Regular Markdown does not support including content from external files, and I'm not aware of any Markdown extensions that add this feature.
You may be able to hack together a solution by concatenating files together, e.g.
01-intro1.md
:
# My title
Some content, check out this example:
```
02-example1.js
:
var foo = 'FOO';
03-intro2.md
:
```
More content.
Generate your presentation file with cat 01-intro1.md 02-example1.js 03-intro2.md > presentation.md
.
For more details, check out this question.