New features

#1565 - Added support for external account binding, meaning that an ACME registration can be linked to a pre-existing account created with the service provider. This can be used by the service provider to provide extra services which cannot be delivered to (semi)anonymous users, e.g. organisation validation or paid certificates. The ACME registration is linked to the external account by means of an HMAC key, which can be provided through the interactive menu upon first use of the service, or from the command line using --eab-key-identifier and --eab-key. Requested by @kgeis.
#1626 - Added a new DNS validation plugin for DigitalOcean, contributed by @Skulblaka.

Enhancements

#1485 - @InKahootz updated the Azure plugin to support alternative endpoints, e.g. to be able to use the Germany, US Goverment or China regions, but presumably this should also work for Azure Stack instances at various service providers.
Audun Skjelnes contributed an example script for KEMP Loadmaster.
When importing renewals from version 1.9.x, by default those will be configured to use both the PemFiles and the PfxFile store plugins to the certificate cache folder to make their behaviour more similar to that of the legacy release. This should make the upgrade experience smoother.
Only the most recent ten history entries will be displayed in the "Show details" menu of the renewal manager.
The process of writing renewals back to disk is now more reliable, due to a sanity check on the JSON serializer and working with a backup/replace mechanism instead of a direct overwrite of the previous file.
#1618 - When an error occurs early in the startup, the process doesn't immediately exit anymore, making errors messages like corrupted settings.json more easily visible to a user working from the Windows desktop.
#1628 - The PemFiles store plugin will now output an extra file that contains only the chain certificates, so excluding the actually issued certificate. This improves it's usefulness for some software like Apache Tomcat. Requested by @ShaynaFishman.

#1533 - A bug introduced in version 2.1.9 caused the http-01 selfhosting validation to fail in certain conditions, thanks to @Gachpen, @mtnhomes and others for reporting this.
A bug introduced in version 2.1.9 caused only the first email address in the list to recieve notifications.
#1614 - Fix a crash when an acme-dns registration cannot be confirmed due to DNS failure, thanks to @LumKitty for the report.
#1620 - Fix a crash when using multiple store plugins of the same type, reported by @srishmawi.
#1625 - Fix an ugly warning message when trying to check if a non-existing folder is empty, reported by @djmcfar.
#1631 - Change the menu shortcut for "Analyze duplicate renewals" because it conflicted with "List all renewals", as noticed by @SistemasMabisy.
#1623 - Improve support for non-English languages, thanks @fatihkizmaz
#1623 - When providing invalid input for --installation, the error message would report a problem with --store instead.
Fix cache bug using DNS domain substitution (CNAME's) in multithreaded mode.