Allow providing a registration email

[mithrandi]

Currently all ACME registrations are anonymous. Let's Encrypt doesn't have a problem with this, but some other future ACME CA might not allow this, and providing an email address with your registration enables you to receive certificate expiration warning emails from Let's Encrypt, which are useful if something goes wrong and you're not watching your logs closely to see the issuing failures from txacme.

[mithrandi]

#97 provided the low-level support for this, but the endpoint parameters still need to be extended to support passing an email address.

[glyph]

I'd love to transitively put support for this into https://github.com/glyph/lancer.

[mithrandi]

@glyph Pass the email parameter to AcmeIssuingService here: https://github.com/glyph/lancer/blob/8d065def9bfd19b8b10b9e9c1f7236cd6019f0cb/lancer/_impl.py#L76

[glyph]

Rad. Thanks.