`--doc-html FILE`: the HTML documentation of the selected scenarios.

One document: a clickable table of contents, one section per feature, one
anchor per scenario. It is written from the test-plan, so *after* the filters -
what would have run is exactly what gets documented. For that the plugin must
come after `:kaocha.plugin/scenari-tags` in `:kaocha/plugins`; kaocha's own
filters (`--focus`, `--skip-meta`) always run first.

Nothing is executed: this is static documentation, not a run report. The
suites are marked `::testable/skip` once the file is written.

`--doc-report FILE` writes the same document after the run, annotated with
each scenario's and step's result.

`--dry-run`: checks that every step of the selected scenarios has a glue,
without running anything.

The glue is resolved at parse time (`scenari.v2.core/pickle-step->map` sets
`:glue nil` when nothing matches), so everything is already in the test-plan:
walking it is enough. Without this an undefined step only blows up when run,
on an `(apply nil ...)`, *after* the previous steps and their side effects.

Non-zero exit when a glue is missing, with the skeleton to paste for each.
Like `scenari-doc`, list it after `:kaocha.plugin/scenari-tags` to check only
what would have run - the walk is scenari-doc's, both plugins read the same
filtered tree. It cannot be combined with `--doc-html` / `--doc-report`.

`--tags "@a and not @b"`: filtering on a Cucumber tag expression.

kaocha can only express an OR (`--focus-meta`/`--skip-meta` hand their list to
a `some`), and its focus is dropped for the whole subtree as soon as a node
matches - a tagged feature therefore runs all of its scenarios. Here the
expression is evaluated scenario by scenario, as Cucumber evaluates it pickle
by pickle.

Only scenari suites are concerned: `--tags` leaves a clojure.test suite
running. To run the features only, combine it with `--focus`.