Extra shell args/options for manage commands

Thank you for this useful plugin.

It would be a very useful feature if we could provide extra arguments to commands like `dump_sql`, `restore_sql`, etc.

For example we would like to use sql format for backups, but we can't provide such useful arguments like `-c` or `-Z9` to `dump_sql` command. 

Then we could use something like:
`dokku:dump_sql app  > app.gz --extra-options="-c -Z9"`
