backup: Add --iexclude-file option

[aawsome]

What does this PR change? What problem does it solve?

closes #2427

Was the change discussed in an issue or in the forum before?

see #2427

Checklist

• I have read the Contribution Guidelines
• I have enabled maintainer edits for this PR
• I have not added tests for all changes in this PR; did not find tests for --exclude-file nor --iexclude
• I have added documentation for the changes (in the manual)
• There's a new file in changelog/unreleased/ that describes the changes for our users (template here)
• I have run gofmt on the code in all commits
• All commit messages are formatted in the same style as the other commits in the repo
• I'm done, this Pull Request is ready for review

[MichaelEischer]

LGTM. Thanks!

I've made a few small changes: The help text of the backup command now prints file as flag-value name instead of stringArray. I've also updated the help text in the documentation along with rewording the changelog message a bit.