Allow disabling the rendering of the cause chain

[LukeMathWalker]

Hi!
I have been using miette for a compiler-like project recently - I have been loving it so far, but I am facing a dilemma.

I'd love to have the fancy-like features for the user-facing reports, but I don't want to expose my users to the cause chain of the top-level error - it would leak implementation details that they should not care about.
As far as I can see, there is no way to suppress the rendering of the cause chain; I can either render the top-level error message (with no source/labels/helps) via Display or I can get the rich error but I end up leaking lower-level error details via Debug.

Would adding a new knob to MietteHandlerOpts be a welcomed addition?

[LukeMathWalker]

It turns out it was quite straight-forward to sketch it out, so I submitted a PR. Feel free to reject it if you don't think this is a desirable configuration option.