Added possibility to modify twig templates in own theme.
Theme templates can be overridden by placing them in plugins/[theme]/templates.
Plugin-templates can be overridden by placing themb in plugins/[theme]/templates/plugin/[plugin]/
Merge branch 'master' of https://github.com/datamintsGmbH/piwik into datamintsGmbH-master
Merge branch 'datamintsGmbH-master'