Replace `populate_next_and_previous_page` with `populate_siblings`

This commit removes the option to sort by order and also removes
`page.next` and `page.previous` variables.  Instead, pages can be sorted
by two methods `date` and `weight`.  The Tera `reverse` filter will
reverse either of those sorts, so the old `order` behavior can be
achieved by using the `reverse` filter with `weight`.

In place of the `previous`/`next` variables, this commit adds the
`page.earlier`/`page.later` variables (which are set when the page is
sorted by date) and the `page.heavier`/`page.lighter` variables (which
are set when the page is sorted by weight).  These variables have the
advantage of not having confusing semantics when the `reverse` filter is
used.

Closes #275

Closes #272

Closes #259

Closes #238

* Add ignored_content to the Config structure.
* Use the GlobSet crate to parse the glob patterns into a matcher, which
  is created once at program initialization. If there are no patterns in
  ignored_content, an empty globber is created, which excludes no files.
  This is consistent with the existing behaviour of Gutenberg, before
  this feature was added.
* Bail if there are any errors in the glob patterns.
* Add a call to the globber in page.rs to actually do the filtering.
* Update documentation.

A note on the Config structure
------------------------------
* I had to remove the PartialEq derive from the Config structure as it
  does not work for the GlobSet type. No harm is done, Config does not
  need to be PartialEq anyway, since there is no need to sort Configs.
* The implementation follows the pattern of the existing config settings
  in that it uses an Option<...>. This would appear unnecessary, in that
  an empty vec could be used as the default, but it appears to be needed
  by the TOML parsing. A better approach would be to use a separate
  SerializableConfig and map to/from a Config struct. This would also
  allow the elimination of most, if not all, of the other Options in
  the Config structure, but that ought to be another PR.

Fix #215

Closes #210

Useful if you are, e.g., generating an index of blog posts.

And some tweaks as I write the docs