Add <=tag some > capture variables

[rwxrob]

Often a variable needs to be captured so that it can be used later in the same rule. This syntax begins with a regular capture angle bracket, but is immediately followed by a colon and an identifier that can be used only during the rest of that rule definition. Use of capture variables should be discouraged when creating an actual rule is clearer, but without capture variables we can never create rules for opening tokens of varying length and match that same token later (~~~ in Markdown, etc.).