Allow to pipe some command before DB dump import

[michalbiarda]

One might want to add for example: | sed 's/utf8mb4_.*_ci/utf8mb4_general_ci/g'

I suggest to allow users to add array of commands that will be piped. For example:

configuration:
  import:
    modify-dump:
      - "sed 's/utf8mb4_.*_ci/utf8mb4_general_ci/g"
      - "something else"

Here's the method responsible for generating import command: \Driver\Engines\MySql\Import\Primary::getImportCommand.

Please remember to document this feature in README file.