Add more aliasing support

[terricain]

An MR to fix a few things, will tag people to check over some stuff in a bit.

[terricain]

If people are able to test this without me having to do a release, I'd appreciate it. Am not finished but this branch currently should be in a working state.

@snigel - This should fix your issues on #30. I reckon it can still be improved more, but it should do until then.
@mdhowle - Have overhauled the DNS delegation docs section and added some more features, you mind having a read over it to see if it makes sense?
@theodiem - You mentioned in #20 about giving certbot only permissions to specific records and not deleting them during cleanup, this allows this somewhat. Currently you'll need to add an entry for each record you wish to not delete. If I can summon the will to redo how the config works, I might look into making this much nicer.

[theodiem]

@terrycain I had a quick look into the commit changes and looks great. I will get some time this week to test it and provide feedback regarding the Azure DNS and its per-record permissions.

[mdhowle]

I haven't had time to look at the code portion, but the documentation looks okay to me.

If you want to give some external resources, these links could be helpful:

• https://letsencrypt.org/docs/challenge-types/#dns-01-challenge (fourth paragraph)
• https://www.eff.org/deeplinks/2018/02/technical-deep-dive-securing-automation-acme-dns-challenge-validation

[terricain]

@snigel @mdhowle @theodiem These changes are now out on 2.2.0b0 on PyPI and on the snap edge channel. Assuming I get no complaints, I'll release 2.2.0 officially in a week or so.