Rel 0.9.18 version bump & CHANGELOG update

Add idna encode/decode helpers

Implement a sketch of --output-provider support for dump

AxfrSource and ZoneFileSource: support multi-value PTR records

Implement black formatting

Co-authored-by: Viranch Mehta <email@viranch.me>

Remove repo-specific FUNDING.yml

Support dotted subdomains for subzones

This confirms that in addition to the recently added support for dotted
subdomains that subdomains that are not dotted are supported.  From
RFC1034 Section 3.5 this would be a <subdomain> that contains a single
<label> without <subdomain> recurring.

Per PR review, use list comprehensions as they are prefered in py3 over
use of filter.

Add comments to describe the building of the zone tree.

Currently if there are two zones configured;
- example.com.
- delegated.subdomain.example.com.

When an NS record is created in example.com.yaml as such:
delegated.subdomain:
  type: NS
  values:
  - ns1.example.org.

The NS record for delegated.subdomain.example.com cannot be created as it
throws an exception:

octodns.zone.SubzoneRecordException: Record delegated.subdomain.example.com is under a managed subzone

Additionally, all records other than NS are rejected for
subdomain.example.com..

This is caused by zone_tree being the result of all zones split on '.'
and being added to the tree, even if a zone does not exist at that
point.

To support records where a subzone is dotted, the the map is built such
that each node represents the subdomain of the closest subzone.

Before:
{"com", {"example": {"subdomain": {"delegated": {}}}}}

After:
{"example.com": {"delegated.subdomain": {}}}

Fixes: #378

The is not a zone between delegated.subzone.unit.tests. and unit.tests.,
however we get a delegated subzone error.  This modifies the tests to
succeed with the added record, however the tests fail as it incorrectly
throws the managed subzone error.

Change the name of delegated.subzone, and the tests will pass cleanly.

Add documentation for `include_meta` config key

Update README.md to include documentation for the `include_meta` key in the `manager` section of the config.

Fix typos

Updated link to providers in README.md

The supported providers table was renamed in 9b79c98a09, this would fix the link to the table.

SSHFP support for Source AXFR/Zonefile

Feature: TLSA record

Rel 0.9.17 version bump & CHANGELOG update