Improve block naming interface

[ixjlyons]

Mandating that every block class take a name kwarg is a pain, but assuming the name is the class name could lead to conflicts if a block type is reused (definitely a possibility with e.g. Filter). It only really matters in the context of a pipeline, but mandatory naming when constructing a pipeline is also a bit painful. The scikit-learn solution wouldn't work well because of how pipeline structure is specified in copper.

[ixjlyons]

Candidate solution: specify the pipeline structure using block names then supply the block objects in a dict by name.

p = copper.Pipeline(
    ['block1', 'block2', (['block3', 'block4'], 'block5'), 'block6'],
    {
        'block1': SomeBlock(),
        'block2': AnotherBlock(),
        ...
    }
)

Note this would make solving #2 pretty straightforward as well.