Add semver sort option for tags listing and delete with -keep#29

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KarolBuchta wants to merge 2 commits intomlabouardy:masterfrom

KarolBuchta:feature/semver-sort-option

Commits on Jul 31, 2018

Add semver sort option when listing tags as well as when using the keep flag when deleting. This ways it is possible to retain the last X versions. Used an exception for 'latest' tag.
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Rename method for obtaining the comparison function, improve description of command.
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