Feature idea: Allow some code a block to be retained/uncommented

While #2 addresses the idea of leaving comments in code block as a guide to people completing the notebook, sometimes it may be desirable to have some working code retained, or a skeleton of the working code included, and _not_ commented out. To accomplish this, it may be nice to have a way to designate lines that should be retained.

[`learnr`](https://rstudio.github.io/learnr/index.html) accomplishes something like this with [separate solution blocks](https://rstudio.github.io/learnr/exercises.html#Hints_and_Solutions), but that seems too heavyweight for this application, and potentially allows drift between the exercise block and the solution.

In the Jupyter world, [`nbgrader`](https://nbgrader.readthedocs.io/) instead uses [specific comments](https://nbgrader.readthedocs.io/en/stable/user_guide/creating_and_grading_assignments.html#autograded-answer-cells) to designate the start and end of a solution section, retaining the code outside those comments, and replacing only the code within the comments with a code stub:
`### BEGIN SOLUTION` and `### END SOLUTION`. This solution has the advantage that the solved code chunk will run in the original document as a check that it is correct behaving as intended.

Under this scheme code of the following form:

```
### BEGIN SOLUTION
plot(data)
### END SOLUTION
```

would become:

```
### YOUR CODE HERE
```

or similar.

One other alternative is an indicator that a specific line should be replaced by commenting at the end of the line that would indicate that particular line should be replaced.

This could look like this:
```
plot(data) ### SOLUTION
```
to get the same result as above.

One other implementation might include a way to strip a commented line. Something like this:

```
### BEGIN SOLUTION
plot(data)
#> plot()
### END SOLUTION
```

could turn into:

```
plot()
```

The issue names no files, tests, or entry points. First resolve whether solution blocks, line markers, or stripping commented lines is the intended syntax, then locate the repository's code-chunk transformation path and add tests covering the chosen markers and the resulting exercise code.

Written by the indexing model from the issue text.