New features

• Experimental: Built-in ACME client (GH #3)

Currently supports only dns-01 challenge with a limited set of
providers. See documentation for details.

• S3-backed storage for message contents (GH #304)

• Local sender authorization (GH #268)

• LDAP BindDN authentication (GH #273)

• storage/imapsql: Implement auth_map

• storage/imapsql: Implement delivery_map

This functionality allows imapsql storage backend to be correctly
used with non-email-based authentication providers.
In particular, this unbreaks PAM and shadow modules.

• Implement table.chain module

• Implement table.email_localpart as a helper to strip domain from emails

Improvements

• Implement client timeouts for target.remote and target.smtp
• endpoint/smtp: Add max_header_size

Fixes

• check/spf: Change default action for softfail to 'ignore'
• endpoint/smtp: Allow to change the line length limit enforced by go-smtp
• table/sql_query: Allow to use numbered parameters in queries
• auth/plain_separate: Make configuration directives actually work
• table/file: Allow table to be created without specifying files in inline args
• config/tls: Fix custom loader configuration reading

Removed functionality

• check/dns: Mark require_matching_echo as deprecated
• config/tls: Remove deprecated "tls CERT KEY" syntax
• Remove deprecated 0.3 module name aliases