Add proxy function sg_animate_p

[sgrubsmyon]

This enables users to trigger animations from Shiny, e.g. using Shiny input widgets, rather than having to restrict to using sg_button.

[coveralls]

Pull Request Test Coverage Report for Build 196

• 0 of 6 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.3%) to 40.938%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
R/graph_proxies.R	0	6	0.0%
<!--	Total:	0	6	0.0%	-->

Totals
Change from base Build 194:	-0.3%
Covered Lines:	393
Relevant Lines:	960

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💛 - Coveralls

[JohnCoene]

Looks good thanks.

Do you want to add yourself as ctb to the DESCRIPTION before I merge?

[sgrubsmyon]

Thanks, @JohnCoene. I added myself to DESCRIPTION.