Add Yandex and Google counters

In order to add Yandex and Google counters you should set
counters' ids in _config.yml:

    theme:
       counters:
          yandex_metrika: YANDEX_ID
          google_analytics: GOOGLE_ID

Resolve #35
Resolve #36
Merge pull request #44 from ikalnitsky/theme-improvements

Add Yandex and Google counters