Add mappings to goto previous or next pages

As in issues [#197](https://github.com/akhodakivskiy/VimFx/issues/197), add mappings to:

```
[     goto previous page
]     goto next page
gu   go up one level in the URL hierarchy
gU   go up to root of the URL hierarchy
```

Matching patterns can be changed in Settings page. Defaults are:

```
+  prev_patterns:      'prev,previous,back,<,«,<<'
+  next_patterns:      'next,more,>,»,>>'
```