netiquette.yml: slightly change the "chaining" block format

In "chaining", we combine blocks defined in the "block" section to tell Dave
how these blocks may occur in a valid email.

Using "<foo>" (same syntax as subrule calls in "grammar" values) to mean "the
'foo' block" may incorrectly lead users to think they can use all shiny
features from Perl 5 regexes, which they don't.

So we need another syntax. Not using any delimiter could be less readable.
"{" and "}" are used for quantification, and "(" and ")" for grouping, so
we're left with "[" and "]", which I like here.
add Dave::Email, awaiting for its *-parts specializations
add DaveNull::Email::{Single,Multi}part, better DaveNull::Email design, tests
replace defined-or (5.10 -> 5.8)
(minor) netiquette.yml: no format change, minor simplification

Remove YAML references: needlessly confused those who don't know YAML, and
were useless since "max-width" is inherited from the root everywhere (unless
where redefined).
dist.ini: Data::Dump::Color is recommended, not required + stuff
DaveNull::Email: add the header() method, refactor associated tests
TODO: email modelization is done