This adds a way of specifying where the output of an executable snippet will go, so it's not always shown right below the snippet. This can be useful in particular to place code in another column when the code itself is already long enough. To do this you need to:

• Create a +exec snippet and add a +id:<name> attribute.
• Add a comment command snippet_output: <name>. The output of the snippet will be placed here.

For example:

<!-- column_layout: [1, 1] -->

<!-- column: 0 -->
```bash +exec +id:foo
echo "hello world"
echo "the time is $(date)"
```

<!-- column: 1 -->

<!-- snippet_output: foo -->

Looks like:

The +id property may have more uses in the future but for now this is all it can be used for.

Closes #540

PS: this needs more testing and some better error reporting but I'm fried already.