Self-created meta fields

Long story short: Now you can create your own ~~portals~~ meta tags.  For example,

```rst
 .. custom-meta-tag: Chris is awesome
```

```mako
Everybody knows that <em>${post.custom-meta-tag}</em>.
```

```html
Everybody knows that <em>Chris is awesome</em>.
```

More detailed example should appear [on the `src` branch of my GitHub page](https://github.com/Kwpolska/kwpolska.github.com/tree/src) soon, along with the branch itself.