The converter now maps arbitrary open blocks to LaTeX environments.

See the README, new section 4.3, "LaTeX Environments" for details.
Briefly, a file, "new_environments.tex" is generated which
contains a simple but serviceable definition of a LaTeX
environment corresponding to the given open block.  These
definitions are stored in the file "new_environments.tex"
The user can use this file as a starting point for customized
environments.  Via this mechanism the converter needs to "know"
very little about LaTeX.  

NOTE: I used global variable $latex_environment_names  to accumulate
the block titles encountered by the parser.  There is probably a much
better way, so please feel free to change this.

I like the idea. However, I would be much more comfortable if the environment name was marked explicitly rather than overloading the use of the block title. Something like:

```
[env=foo]
--
blah blah blah
--
```

You could then retrieve the environment from the attributes map.

Other idea is to dedicate the block name "env" for this purpose and then it would be more reasonable to reuse the title since the association is now explicit.

```
[env]
.foo
--
blah blah blah
--
```

Or, instead of the title, you could make it the first role. In my mind, role maps best for this scenario:

```
[env.foo]
--
blah blah blah
--
```

This second approach would require registering an extension as part of the converter.