- bug fix in node_processor, Block#open_process, handle case of nil title
First draft: [env.theorem]
Primitive toolchain: pre and post processor, block processor (env)

The toolchain can now handle the "environment block"

   [env.theorem]

with either the html or latex backend. Note that the second
element of the environment block is variable and not defined
in advance.  Thus one has the usual [env.proposition],
[env.definition], [env.lemma], etc, but also [env.joke],
[env.prayer] ... whatever you need.
Added HTML handler for env, added ClickBlock class