Aliases that have no directory nesting sneakily avoid the code path that ensures the directories exist

Closes #486

Closes #485

Local CSV/TOML/JSON loading Tera function

Close #446
Close #260
Close #478
Close #284
Close #480

Can't remember why it was doing that even
with the comment and the test added in the
commit still pass so...

Fix #462

Switch to an `Option<usize>` for the serialized value of `rss_items`.
This lets us just set a blank value in the configuration and thereby
include *all* items.

This is a backwards-compatible change; it does not affect the behavior
of existing configurations.

Fixes #468. Closes #471.

Fix #122

Fix #205

This allow to use base_url in robots.txt, to reference a sitemap for
example.

Closes #443

Fix #430

Not only is this tidier, but it avoids making these iterators
Rayon-specific.

This offers a big performance bump for large sites:

  small-kb (100 pages)
    before: 530ms
     after: 500ms

  medium-kb (1000 pages)
    before: 2.9s
     after: 1.5s

  huge-kb (10,000 pages)
    before: 150s
     after:   9.1s

Fix #383

Closes #388