Add <=foo ... > !foo syntax

[rwxrob]

Now that code generation is not the primary goal, we can support other rhetorical clarifications in the specification grammar itself, such as capture variables that have scope only for that given rule. As such we can specify things like the problematic fenced code opening tag variations of Markdown.

~~~some
~~~~another
...
~~~~
~~~

Fenced <-- <:tok '~'{3,8} SP? > !tok ClassName tok