Fix email scrubber hash input

[5p2O5pe25ouT]

Fixes #45

Summary

• hash the email local part when using the emails scrubber
• keep the existing behavior of replacing the local part and preserving the original domain
• add coverage for multiple emails with the same domain so different local parts no longer collapse to the same scrub hash

Verification

• RED: go test ./scrubbers -run TestEmails -count=1 failed before the fix because alice@example.com and bob@example.com produced the same hash
• go test ./scrubbers -run TestEmails -count=1
• go test ./...
• git diff --check

[coveralls]

coverage: 94.695%. remained the same — 5p2O5pe25ouT:fixbug/issue-45-email-scrub-hash into go-playground:master