[stacktrace] Add support for directly inspecting ex-data

[alexander-yakushev]

Changes made to address clojure-emacs/cider#3804.

1. The op inspect-last-exception now accepts ex-data: true parameter that tells it the user wants to inspect ex-data of the exception directly, not the entire exception. This is used to have separate buttons in *cider-error* buffer for the whole exception and for just ex-data.
2. Replaced a separate dynamic variable manually stored in the session with storing the last analyzed exception inside session's metadata – the same approach as used by inspector middleware.

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• You've updated the README
• Middleware documentation is up to date

[bbatsov]

Looks solid to me!