Implement @pagination_interval

I'm currently working on a scraping a website (sorry, can't share it at the moment) that handles pagination more like `offset` in SQL than using page numbers. Each page displays 20 instances. The first page is `start=1`, the second page is `start=21`, the third is `start=41`, etc. This pull request allows the user to ability to specify a number other than 1 that should be added to `pagination_index` to get the next page. The default value of `pagination_interval` is 1. For this site, setting up Upton to handle pagination would like this:

    scraper.paginated = true
    scraper.pagination_param = 'start'
    scraper.pagination_interval = 20
    scraper.pagination_max_pages = 12001