Command line tool to generate doi.xml files

[mpsonntag]

Analogous to the make-html command line option of the gindoid binary, this PR adds the make-doi-xml command line option.

When provided a datacite.yml file, a URL to a datacite.yaml file or a public GIN repository containing a valid datacite.yaml file in the format GIN:owner/repo the binary will create corresponding doi.xml files.

e.g. ./gindoid make-doi-xml GIN:jgrewe/grewe_etal_synchrony local_datacite.yml https://gin.g-node.org/jgrewe/grewe_etal_synchrony/raw/master/doi_info.xml

[coveralls]

Pull Request Test Coverage Report for Build 856146195

• 0 of 88 (0.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-1.0%) to 20.659%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
cmd/gindoid/main.go	0	13	0.0%
cmd/gindoid/genxml.go	0	75	0.0%
<!--	Total:	0	88	0.0%	-->

Totals
Change from base Build 811656512:	-1.0%
Covered Lines:	401
Relevant Lines:	1941

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💛 - Coveralls