Normalize all stacktrace under the same name and improve IEx errors

Kudos to @pragdave for suggesting some those improvements and
providing an initial implementation in pull request #1752.
Provide a wrapper in CLI for easy stacktrace pruning

As suggested by @pragdave
Remove unused warning
Merge pull request #1753 from mururu/rm-warning

Remove unused warning
Fix indentation on doc example
Merge pull request #1754 from pminten/exunit-case-doc-fix

Fix indentation on doc example