Improve XML attribute snippet, add XML comment snippet

UltiSnips/xml.snippets

@@ -3,7 +3,7 @@ snippet t "XML inline tag" i
 endsnippet
 
 snippet a "XML attribute" i
-${1:attrib}="${0:value}"
+${1:attrib}="${2:value}"$0
 endsnippet
 
 snippet < "XML inline tag" i
@@ -15,3 +15,7 @@ snippet tb "XML inline tag" i
 	${2:content}
 </${1/([\w:._-]+).*/$1/}>
 endsnippet
+
+snippet <! "XML comment" i
+<!-- $1 -->$0
+endsnippet