Run checking: Remove option to generate html

[ceball]

nbsmoke and nbsite unfortunately have various bits of overlap. One bit of overlap is that both can run notebook and produce html.

Initially it was convenient to store the result of nbsmoke's running of notebooks as html, and make that html available for viewing. E.g. travis could be set up to run a project's notebooks and make the rendered notebooks available for viewing. However, we now use nbsite for this.

On the other hand, it's not really a big deal to keep store-html in nbsmoke.

[ceball]

Would be fixed by #31 :)