feat: allow including external code snippets

[mfontanini]

This allows adding snippets that pull in an external source file. Attributes still work like you'd expect. Examples:

A rust file that allows execution:

```file +exec +line_numbers
path: snippet.rs
language: rust
```

A mermaid script that is rendered like you'd expect

```file +render
path: snippet.mmd
language: mermaid
```

Related to #307