Set t_Co to 256 for HTML output

I can now reproduce the problem if I run vim, and `:set t_Co=8` before I do `:SyncopateExportToBrowser`.

The solution is pretty simple: just add `&t_Co` to the list of settings to save, and force it to 256.  This value only applies for HTML output; it doesn't make sense to constrain the HTML colours we can use for an HTML page.

I verified that this commit makes the problem go away.