Add ability for submodule release (diff cleanup)

[hawknewton]

This is #8 with line endings changed to match the original files. The code was written by @jutzig, who deserves all credit.

This change was made without editorialization or modification -- my intention is to cleanup the diff so it may be considered for inclusion.

Thank you.

[buildhive]

Jenkins » m2release-plugin #14 SUCCESS
This pull request looks good
(what's this?)

[imod]

+1
This is definitely a valid use case.

[jenkinsadmin]

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[imod]

Why did you change the lineending? Its impossible to see the actual change now - the chance to get this merged like this is close to zero...