Add a smarty filter for smartypants processing

Example:

```
{{ post.title | smarty }}
```

If `post.title` is `"My blog's first post..."` it will come out as `My blog&rsquo; first post&hellip;`.

Essentially, it's just the SmartyPants part of the `markdown` filter.

Also added some docs covering other filters.

Contains a workaround for a single backticks bug:

https://github.com/vmg/redcarpet/pull/201