Added undo-tree

Added undo-tree, which is a popular package for undoing changes made to a file. It leaves hidden files everywhere, and can litter up a repository if not ignored. 

I have to add `*.~undo-tree~` to every `.gitignore` file in every repository I am working in. As you can guess, this gets very tedious.

Author: anoduck

Global/Emacs.gitignore

@@ -47,3 +47,5 @@ flycheck_*.el
 # network security
 /network-security.data
 
+# undo-tree
+*.~undo-tree~