[MRG] Refactor bootstrap and allow user to pass in custom subsampling function

[thuijskens]

TODO:

• Improve help docs
• Write example that shows how to use complementary pairs bootstrapping
• Allow keyword arguments to be passed to bootstrap_func

[coveralls]

Pull Request Test Coverage Report for Build 40

• 47 of 48 (97.92%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.9%) to 86.765%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
stability_selection/stability_selection.py	25	26	96.15%
<!--	Total:	47	48	97.92%	-->

Totals
Change from base Build 28:	1.9%
Covered Lines:	236
Relevant Lines:	272

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💛 - Coveralls