Support Git Tag Counters

[bastiao]

Add support to count a number of tags per period of time.

The idea is to extend contributions to support a list of git tag counters in range period.

It is expected to work like that:

./bin/contributions gitler --csv-file=repos.csv --export-file=gitcounters.txt

The output should be like:

repositories, number of tags
repo1, 20
repo2, 10
repo3, 0