Make map and types directives analysable

The `map` directive (from ngx_http_map_module and ngx_stream_map_module)
and `types` directive (from ngx_http_core_module) are distinct from
other directives, in that their child directives may be arbitrary
strings that aren't known to Nginx. This causes crossplane to report
unrecognised/misused directive errors when analysing them with `strict`
or `check_ctx` enabled; it also means that no meaningful analysis can be
be performed when `check_args` is enabled because the arity of child
directives is unknown.

Add support for `map`'s special directives (`default`, `hostnames`, and
`volatile`) to the analyser. Recognise that arbitrary directive names
inside a `map` or `types` block are valid and that they accept either
one argument (if used in `map`) or one or more arguments (if used in
`types`).

Fixes nginxinc#101 and nginxinc#103.

AUTHORS.rst

@@ -14,4 +14,5 @@ Contributors
 * Ivan Poluyanov <[email protected]> `@poluyanov <https://github.com/poluyanov>`_
 * Raymond Lau <[email protected]> `@Raymond26 <https://github.com/Raymond26>`_
 * Luca Comellini <[email protected]> `@lucacome <https://github.com/lucacome>`_
-* Ron Vider <[email protected]> `@RonVider <https://github.com/RonVider>`_
+* Ron Vider <[email protected]> `@RonVider <https://github.com/RonVider>`_
+* Chris Novakovic <[email protected]> `@chrisnovakovic <https://github.com/chrisnovakovic>`_