txt: use <w:br/> for line break instead of <w:cr/>

Word does not interpret <w:cr/> element in run as a line break,
contrary to the behavior required by the ISO 29500 spec. Use <w:br/>
element to implement line break when a line feed (‘\n’) or carriage
return (‘\r’) character is encountered in text assigned to a run.
release: prepare v0.7.2 release