feat: re-add html escaping

Merge pull request lukasjarosch#13 from sashamelentyev/feat/dont-panic

feat: dont panic

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