feat(components): Add retainOnceInViewport prop to Viewport.WithPlaceholder

[bevkoski]

This feature is inspired by a Medium comment.

It adds support for keeping a component wrapped by Viewport.WithPlaceholder displayed once it enters the viewport.

With retainOnceInViewport set to false (default behavior), the component will be replaced by a placeholder once it leaves the viewport.
With retainOnceInViewport set to true, the component will be kept displayed.

[coveralls]

Pull Request Test Coverage Report for Build 335

• 0 of 4 (0.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.6%) to 77.211%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/components/src/viewport/withPlaceholder/index.js	0	4	0.0%

Files with Coverage Reduction	New Missed Lines	%
packages/components/src/viewport/withPlaceholder/index.js	1	0.0%

Totals
Change from base Build 334:	-0.6%
Covered Lines:	852
Relevant Lines:	1030

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💛 - Coveralls