Add static HTML output option from slack2html

[merobi-hub]

The separate project slack2html serves a common use case -- publishing an archive on a host -- but, being a separate project, is not very convenient.

This borrows that project's dependencies, CustomFreezer class, and instantiation to add an option to output static HTML to any specified directory. The class is in a new script in slackviewer. The only changes to the html output implementation are the addition of a new command-line argument and a default output directory.

[merobi-hub]

I used it to generate a simple archive site