add version flag getting info from git repository

[ilmanzo]

fix #16

$ ./comics-downloader --version
Build on 2019-05-03_11:47:55 from release v0.10-1-g890eb6a

[coveralls]

Pull Request Test Coverage Report for Build 76

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 78.053%

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Totals
Change from base Build 73:	0.0%
Covered Lines:	473
Relevant Lines:	606

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💛 - Coveralls