Allow multiple "{{PLURAL}}" with i18n.twntranslate method

avoid duplicate code by extracting parts from pywikibot.translate()
and i18n.twntranslate() to "internal" method _extract_plural()

enable tuple or list as parameters and check whether the items
count is equal to the number of plural items found by re.findall()

Some test stuff added.

Change-Id: I6420b22835ec8db9c70806f46bf2efda65944c34
Merge "Allow multiple "{{PLURAL}}" with i18n.twntranslate method"