Is your feature request related to a problem? Please describe.
Currently the replacement string is ***REMOVED*** . In Drupal there are a configuration *.yml files. The secrets are mostly located into them. The asterisk marks (*) are not allowed in a *.yml files as a values. So I need manually surrounded the string by quotation marks like this '***REMOVED***' or find/replace it with another string.

Describe the solution you'd like
It would be great to have an option which could be passed to the command to use a custom string for the replacements. Like this:
gitleaks detect --report-path gitleaks-report.json --replacement-string REMOVED

Additional context
Using in Drupal CMS

cc @zricethezav