Feature proposal: cell output export

Hello, I am glad to see the work which has been done here!

I am using a lot of notebooks, and I would like to automate them, and get specific cells output.
I think the easier way to do so would be to run a notebook using dotnet-repl, identify a cell, and get its raw output as a file.
I have a crude pipeline, and I am looking for a way to improve it, either with existing nugets, or by suggesting a feature.

### The problems
1: a way to give a name or id to a cell
I am talking about a user defined name, not an automatic one like the cell number.

- Using ipynb format, we can have a first line comment such as #id:myFinalOutput, which can be parsed later from the cell source.
- Using trx format, the first line of the cell is used to create a parameter `testName="Cell X: first_line_of_the_cell"`.
- Maybe a magic command would be better?

2: a way to get a raw output of the cell
- Using ipynb format, the output looks like this:
```json
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"0\r\n",
"1\r\n",
"2\r\n",
"3\r\n"
]
}
]
```
- Using trx format, the output looks like this;
```xml

0
1
2
3

```
It's possible to parse the files. But I think it would be better if dotnet-repl outputs directly the raw output to a a file.

### Proposal for one output

The notebook
```csharp
#!cellid myFinalOutput
Console.WriteLine("Hello World");
```

The command line
`dotnet repl --run /path/to/notebook.ipynb --output-path /path/to/output.txt --cellid myFinalOutput`

The output (output.txt)
`Hello World`

As the output is raw, it could also be some html.

### Proposal for several outputs
Managing several outputs for several cells.

The notebook
```csharp
#!cellid myIntermediaryOutput
2 + 2

#!cellid myFinalOutput
Console.WriteLine("Hello World");
```

The command line
`dotnet repl --run /path/to/notebook.ipynb --output-path /path/to/folder --cellid myIntermediaryOutput,myFinalOutput`

- myIntermediaryOutput.txt
`4`
- myFinalOutput.txt
`Hello World`