Pagination support

* Added module Hakyll.Web.Paginator with functions
  - buildPaginator[With] - building paginator for specified set of files
  - paginatorRules - make us able to set rules for each pagination page (similar to tagsRules)
  - renderPaginator[With] - rendering of paginator menu; standard menu will rendered like: << < _2_ _3_ 4 _5_ _6_ > >>, providing navigation links to first, previous, next and last pages and some set of adjacent pages.
  - paginatorFields - provides context for "firstPage", "prevPage", "nextPage" and "lastPage" fields.