Add prefix and new Addins

[statnmap]

• I think that renaming the current Rmd is still useful in the addins list. So I propose to add it back but in a separate addin from the "select File" one.
• I added the possibility to define myself the prefix of the chunk name as my very long filenames take too much space in my chunk names. The prefix proposed is pass through the clean_latex_special_characters() to be sure everything is good.
• I updated the unit test accordingly
• As I was in the Addins files, I added the "choose a directory" feature.

The 3 addins:

The interface to choose for "prefix" when using Addin for the "Current Rmd"

[coveralls]

Pull Request Test Coverage Report for Build 95

• 5 of 11 (45.45%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-4.6%) to 90.164%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
R/name_chunks_addin.R	0	6	0.0%
<!--	Total:	5	11	45.45%	-->

Totals
Change from base Build 94:	-4.6%
Covered Lines:	110
Relevant Lines:	122

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💛 - Coveralls