#25 add a tag for hard line breaks

[hhhcj]

Fix issue #25 . {{HardLineBreaks}} will be replaced by <w:p></w:p>, besides <w:br/>, it's hard line breaks.

docx1.Replace("document", "line1{{HardLineBreaks}}line2", -1)

And I also update const NEWLINE = "<w:br/>" with const NEWLINE = "</w:t><w:br/><w:t>", because when I convert this docx to pdf , it will not be recognized as new line, unless I move <w:br/> outside the <w:t>.