Is your feature request related to a problem? Please describe.
If you forget to include the --report-format argument, it defaults to json. This can be confusing if your report file is another format, e.g.:

$ ./gitleaks git . --report-path test.csv

Describe the solution you'd like
If no value is provided for --report-format, check the file extension of report-path and see if it matches a format.

Describe alternatives you've considered
N/A

Additional context
N/A

cc @zricethezav