Allow inclusion of all items in RSS feeds.

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.

Slotmap refactor

Fix #122

Fix #205

* Replace hard-coded directory slash in serve output

Feat: inject config in robots.txt context

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

Fix some clippy warnings

Closes #443

Allow to disable colors

- unless forced, only use colors in tty
- support CLICOLOR and NO_COLOR vars
- switch to termcolor crate to handle output coloring

Fix #430

Replace fold/reduce over Result::and with collect

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

Use a HashSet for detecting orphan pages

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