Various fixes

(I'm not sure if I should have split out these fixes into their own issue, or should they be combined like this into a single pull request.)

**Various fixes**

Wordpress export files contain excerpts which should be extracted into the YAML frontmatter.

The Sitemap view did not remove `index.html` from the end of the URL before looking up `self.mapping`, therefore will always fail to match any entries unless they ended with `/index.html` (which should not be the case).

The "tagify" function was designed to work on a list, but the Link object does not hold additional information, thus tag clouds are constructed manually which has to hardcode the tag view path like this <https://github.com/greatghoul/greatghoul.github.io/blob/source/theme/tags.html#L8>. With this fix, `tagify` can be used to construct a single tag link like so `{{ tagify(tag.name).href }}`.

Lastly, tags passed to the tag view have been `safeslug` before it reaches the template, thus the template is never able to display the original tag. The original tag name should at least be passed verbatim so that the template can display the original tag name to the user, and if required to construct a link can use `safeslug` in the template.