Implement Series macro.

Macro described in https://github.com/gollum/gollum/issues/910. It needs tests and probably refactoring -- please have a look.

@SkyCrawl: I hope this is approximately what you had in mind. `<<SeriesStart(prefix)>>` and `<<SeriesEnd(prefix)>>` can be used when there *are* previous or following pages in the subdirectory, but the user does not want these to be included in the series.