Add BasePipelineItem initialization function

[immerrr]

I've run into situations when there is a need for a quite heavy initialization function for a pipeline item, which should be executed only once before functioning.

Without an initialization function to be executed before the pipeline item is being used first, that initialization either goes to the constructor and thus slows down configuration reading/loading/validation, or to the batch processing function including a "is_initialized" flag to be checked for each batch.

[eliasdorneles]

Hm, yeah, the initialization is currently coupled with the configuration reading/loading/validating.

Adding a method can be a decent solution, I have to think about it a bit more.