Pipelines -- refactoring core concept, source customization selection for ppl items

[nicolay-r]

#535 related

• Now each pipeline item may have src_key and src_func.
  There is the only parameter for the pipeline -- is a context.
  Using extra parameters mentioned above we extract the actual input for the pipeline item from context. We are still provide the whole context alongside with the input element for each item just it case.
• Provide input feeding support in launcher. (901c2ae)
• Pipeline.run might be just a concept of launchers, there is no need to combine storage of items with run operation #540