Add :fixed_page_links option

When set to true, the number of links displayed (including gaps) will always equal 3 (first, current_page, last) + 2 (to allow for gaps) + 2*inner_window + 2*outer_window.

`3 + 2 + 2*inner_window + 2*outer_window` is the maximum number of links displayed, as illustrated in the example below.
```
[<] [first] [outer_window] [...] [inner_window] [current_page] [inner_window] [...] [outer_window] [last] [>]
```

This means that the width of the pagination bar does not substantially change as a user browses through the pages.

Do you think this is an option people would be interested in?