This PR contains a few refactorings that make the SlimdumpCommand, DumpTask and Dumper classes and method signatures more straightforward:

• By creating the MysqlOutputFormatDriver in SlimdumpCommand, we can pass it the necessary information from command line switches directly. No need to pass-through these flags and settings through other classes/methods.
• Separate the OutputInterface given to MysqlOutputFormatDriver (for writing out SQL) from the OutputInterface passed to Dumper (where it is used to output progress information)
• Use a NullOutput for the progress-related OutputInterface instead of passing a $noProgress boolean through several methods. Saves us a bunch of if-conditionals.
• "Extract method" refactorings for some meaningful chunks of code