handle nested placeholders

add placeholder validation; introduce validPlaceholders in ParsePlace…

…holders to not modify the iterating slice (which def will cause issues)

cleanup and refactoring; renamed TextRun to TagPair

remove deltaLength from r.fragmentsFromPosition() call

gofmt

fix license link

add simple Replace test to verify that the replaced docx is actually …

…valid xml

introduce IDs for runs and fragments; fix fragmentsFromPosition bug w…

…hich caused the function to skip a fragment in specific circumstances; added some comments; did some refactoring