add `subdomain_matching` param to `Map`

[davidism]

If server_name is passed to Map.bind_from_environ, and a subdomain is not passed in (as usually it's not possible to know it), it will try to detect the subdomain by removing server_name as a suffix from host. This restricts routing to only the configured server_name, even if we don't have any subdomain-based routing rules.

Move Flask's subdomain_matching param into Map (where host_matching is already). bind_to_environ should not extract a subdomain unless it's enabled.

[davidism]

server_name should also be ignored if host_matching is enabled, it should only be used for subdomain_matching.

[davidism]

default_subdomain should be used if it's set and server_name is not a suffix of Host. Currently "<invalid>" is always used, and I'm not sure this is correct since it causes wildcard subdomain="<name>" rules to still match, even though the comment indicates it should 404. So maybe evaluate removing "<invalid>" as well.

Or maybe it shouldn't. Maybe knowing it's not a valid subdomain, rather than using the default, is more useful? "<invalid>" isn't mentioned anywhere in the docs, and is only present in three tests that don't actually test routing to it, only that it is what is set on the adapter.