Clean up templates and unify template metadata

[mntn-xyz]

This does several things:

• Removes unused and unavailable metadata from the RSS template
• Passes site config data to index templates
• Updates the default index template data to use the same names as other templates (breaking change, .PostData is now .Posts in main index) - note that index/top template code is now mostly identical, so it should be possible to consolidate this code
• Update default index template to use the site title if present and to skip printing dates if unavailable
• Update default single page template to use the page link if no title is present and to skip printing the post date if no date is present

With these changes, users should be able to run gmnhg out of the box with no configuration and get a reasonable default site, with their site's title reflected in the main index. All pages will render OK even if they have no title or date set.

[mntn-xyz]

Done, thanks for the reminder on godoc... I'll get used to it eventually :)