Add the ability to skip current processed item on failure

Add the ability to skip current processed item on failure, if `skipItemOnFailure`attribute is set to `true` `\Ddeboer\DataImport\Exception` will be catched and logger be called with exception message.

The logger of `Psr\Log\LoggerInterface` interface can be passed optionally to the workflow constructor.

What do you think about this proposal ?

I you agree i can write some tests and docs.