1.1.0: UttPipe can add checkpoints

1. Implement Operator __eq__ : Operators can compare.
2. Refactor common tests for Operators: more extensible to add test function.
3. Add Lowercase Operator: Convert characters to lowercase.
4. Add checkpoints: uttut pipe can output intermediate result by adding checkpoints.
   For example,

>>> from uttut.pipeline.pipe import Pipe
>>> p = Pipe()
>>> p.add('op_1', checkpoint='result_of_1')
>>> p.add('op_2')

>>> _, _, _, _, intermediate = p.transform(...)
>>> intermediate.get_from_checkpoint('result_of_1')
# output the intermediate result of op_1 including output_sequence, entity_labels