Chessboard

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* Validate CLI user inputs and provides hints.
* Abandon branches of the search space as soon as possible. Closes :issue:`3`.
* Deduplicate per-kind piece group permutations early. Closes :issue:`7`.
* Add ``--silent`` option to skip displaying of all board results in ASCII art.

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* Refactor solver to deduplicate positions by kind (combination) before
iterating the search space by brute force (cartesian product).

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* Display results in unicode-art.

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* Add Knight model.

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* Add Rook and Bishop models.
* Allow overlapping but non-threatening territory of pieces to co-exists.

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* Add project status badges.
* Enable continuous integration metrics: build status, coverage and code
quality.
* Fix index to position computation in non-square boards.
* Remove restriction on board dimensions.
* Unit-tests result sets produced by the solver.