feat(enforceStep): Add an enforceStep option (defaults to true)

[ValentinH]

Add an enforceStep option (defaults to true) to force the value to be rounded to the step, even when modified from the outside.

As requested in #236

Demo: http://jsfiddle.net/v7zx09ym/

[codecov-io]

Current coverage is 100.00%

Merging #246 into master will not affect coverage as of ec0fa62

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Review entire Coverage Diff as of ec0fa62

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