Add previous_page and next_page methods to paginator

[wout]

Purpose

Adds previous_page and next_page methods to Lucky::Paginator.

(Sorry for not opening an issue first, but this seems like such an obvious addition).

Description

Currently, there are only the path_to_next and path_to_previous methods, returning a path including all query params. There's no way to get the next/previous page number other than calculating it based on the values returned by the paginator.page and paginator.total methods.

I'm working on an API and I need pagination with the full url and without any additional query parameters that may be present. To build the url, I need the previous and next page numbers rather than the paths.

Checklist

• - An issue already exists detailing the issue/or feature request that this PR fixes
• - All specs are formatted with crystal tool format spec src
• - Inline documentation has been added and/or updated
• - Lucky builds on docker with ./script/setup
• - All builds and specs pass on docker with ./script/test

[jwoertink]

Looks good to me!