Provide a way to customize pretty printing

When expect notices an error, it outputs an Expected: Actual: Which: block of text.

The Actual: line, however, wraps the value in angle brackets.

Sometimes, the value I want to print isn't a literal value, it's just an English-prose vague description of the value (the value itself being a gigantic tree that takes pages and pages to print and that isn't useful to print in this context).

This is a feature request to have a way to provide a pretty printer for values, e.g. by implementing a particular interface, or by having the matchers provide the pretty printer for the value, and allowing this API to omit the angle brackets.

Start at the expect failure output that assembles the Expected/Actual/Which block and trace how values are formatted. Compare the possible customization points described in the issue, then define tests for prose descriptions and for omitting angle brackets; done means callers can customize pretty printing for non-literal values.

Written by the indexing model from the issue text.