Way to hide exec input

[jcushman]

It would be cool to be able to hide the exec input, so the output is recreatable but the input isn't shown in the render. My usecase is a tutorial doc that shows the contents of a file and then how to process it:

    ```bash
    echo '--- snapshot-a/story.txt ---' && cat test-vectors/single-file-modify-text/snapshot-a/story.txt && echo && echo '--- snapshot-b/story.txt ---' && cat test-vectors/single-file-modify-text/snapshot-b/story.txt
    ```
    
    ```output
    --- snapshot-a/story.txt ---
    Line 1
    Line 2
    Line 3
    Line 4
    Line 5
    
    --- snapshot-b/story.txt ---
    Line 1
    Line 2 revised
    Line 3
    Line 4
    Line 5
    Line 6
    ```
    
    ```bash
    ./target/release/binoc-cli diff test-vectors/single-file-modify-text/snapshot-a test-vectors/single-file-modify-text/snapshot-b
    ```
    
    ```output
    # Changelog: test-vectors/single-file-modify-text/snapshot-a → test-vectors/single-file-modify-text/snapshot-b
    
    ## Substantive Changes
    
    - **story.txt**: 2 lines added, 1 removed
    
    ```

Having echo '--- snapshot-a/story.txt ---' && cat test-vectors/single-file-modify-text/snapshot-a/story.txt && echo && echo '--- snapshot-b/story.txt ---' && cat test-vectors/single-file-modify-text/snapshot-b/story.txt in the tutorial file is confusing and reads as noise. The goal is to hide it but still be able to regenerate the file contents right afterward.

The best I've thought of is it the parser could handle comments, if like

<!-- ```bash ... ``` -->
```output ... ```

was recognized as showboat output and regenerated.