Bug 11278: Followup for customize command line parameter

Bug 11278: Followup for customize command line parameter

The initial patch for this bug did not include a specific command line
option for customization. If a module LocalChanges.pm existed, it would
be used without asking.
This patch adds a command line option enabling the customization option
and offering the extra possibility of using another module name. If no file
name is passed, we default to LocalChanges.
Without the -custom option, behavior is as it was.
Also some POD lines are added to document the feature.

Test plan:
[1] Make a LocalChanges.pm in migration_tools. Verify that it is not used,
    if you do not enable the -cust parameter.
[2] Run the script again with -cust. Verify that it is called now.
[3] Copy LocalChanges.pm to Whatever.pm. Make some change. Run with
    -cust Whatever and verify that the new module is used.
[4] Copy Whatever.pm to another dir, make some change. Run with -cust and the
    full name. Verify that the latest change was used.
[5] Run without any option. Check the pod documentation.

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>