Make SQL definer replacement optional

[p-makowski]

Magerun pull-request check-list:

• Pull request against develop branch (if not, just close and create a new one against it)
• README.md reflects changes (if any)
• phar fuctional test (in tests/phar-test.sh)

Summary:
Even when magerun db dump command is executed with --only-command option a piped definer replacement is added to its output. In some scenarios it is a bit not intuitive or undesirable, e.g. when dump is imported on the same database or --only-command output is further processed in separate script.

Changes proposed in this pull request:

• Added flag keep-definer to allow a pure mysql command to be outputted or executed.

[cmuench]

@p-makowski Hi my old friend :-)
Thanks for contributing.

[p-makowski]

Thanks @cmuench
Long time no see! I hope to be your guest more often here :)