Fix content parser's 'text' rule to use the correct production

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

Fix #462

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