wdte: add a way to execute a chain section regardless of an error

[DeedleFake]

As of #168, it's now possible to 'catch' errors in chains using -| as a chain operator. What it's not possible to do, however, is to cause a section to be used regardless of whether or not an error happened. A 'finally', essentially. This would be useful for, for example, opening a file, writing to it, and then closing whether or not there was an error.

Two likely possibilities:

• Cause ignored sections to do this. This would be useful for the above example, but might not be useful in some other cases.
• Add another chain operator, such as -|>, which would do this. I'm not particularly a fan of this one, but it's definitely possible.