Move away from using global app env for pipeline module config

[Ziinc]

Pipeline config should be localized to the specific pipeline.
Benefits:

1. Reduces clashes (when more middlewares/pipelines get built)
2. makes parameter declaration clearer
3. makes pipelines more "functional" and reusable
4. use of Application.get_env usage within pipeline modules makes things less clear when declaring pipelines.

Related to #20 , would pave the way for adding logic into a pipeline (instead of having a fat pipeline module)

Proposed api:

• Tuple definitions would only be required for modules that require configuration
  • at the pipeline level, they could throw an error when checking for parameters, or use default values.
• a pipeline module can also be passed directly when no parameters are needed

For example:

pipelines: [
  ....
  MyCustom.Pipeline.CleanItem,
  {Crawly.Pipelines.Validate, item: [:title, :url] },
  {Crawly.Pipelines.DuplicatesFilter, item_id: :title },
  Crawly.Pipelines.JSONEncoder
]

Besides adjusting existing built-in pipelines, this proposed change would also require the adjustment of Crawly.Utils.pipe to check for tuple definitions.

[oltarasenko]

I kind of like this idea!

[Ziinc]

I'll open a PR for this over the weekend