Use of tag use

Deep cloning nodes that should be used before normal document parsing.
Update parser.js

Moved in a separate function.
Tried to fix sneaky tabs and spaces.
Update parser.js

Changed getElementsByTagName with queryselectorall, allow to go just on "use" node , but all of them ( getElementsByTagName doesn't , it select just first level of child.
Removed tagname check now useless.
Merge pull request #1425 from asturur/use-tag-use

Support use tag