Complete refactor & rework of how validation is set up and overall improvements

- adds lenient flag to Record.new, problems during validation are just
  warnings if it's true
- target populate calls during the plan phase pass lenient=True
- make all of the provider.populate call logging consistent including both
  target and lenient
- add source=self to Record.new in a few places that were missing it

This is with an eye toward expanding it in the future both in terms of what it
checks and to add the ability to ignore things. This commit does not intend to
change any validation. It only reworks the flow and improves the error
messaging.

Switch to an explicit SUPPORTS setup

Natural sorting improvements

Rework _Route53Record to avoid a bunch of hacks

Quick-fix disable ALIAS for Route53 & Cloudflare

Cloudflare could potentially support it, but their details are different enough
that i'll need to be looked into specifically later.

Better to use something real/tested and less likely buggy/limited.

Skip planning (and populating) zones without elible targets

docs: Fix small typos

Alias support

Add support for ignored records.

Escape unescaped semicolons coming out of Route53

Fix zone-level always-dry-run functionality

Thanks @offmindby!

```yaml
ignored:
  octodns:
    ignored: true
  type: A
  value: 1.2.3.4
```

DnsimpleProvider updates delete before create, or else errors thrown

NS1 does not support SSHFP

Supports ALIAS for Dnsimple, Dyn, Ns1, and PowerDNS. Notes added to readme about
some of the quirks found while working with them. TTL seems to mostly be
accepted on ALIAS records so it has been added back, what it means seems to vary
across providers, thus notes.

Fix NS1 provider name

Update example class name from "octodns.provider.nsone.Ns1Provider" to working "octodns.provider.ns1.Ns1Provider".

Update README to include new NS1 Provider