add addin

[maelle]

cf #7

Very basic add-in, only works for the active document.

[maelle]

It makes rstudioapi a dependency.

[coveralls]

Pull Request Test Coverage Report for Build 27

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-1.1%) to 96.629%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
R/name_chunks_addin.R	0	1	0.0%
<!--	Total:	0	1	0.0%	-->

Totals
Change from base Build 25:	-1.1%
Covered Lines:	86
Relevant Lines:	89

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💛 - Coveralls