[FR] JMAP support

[ericschdt]

JMAP is the developer-friendly, open API standard for modern mail clients and applications to manage email faster. There's also already a Android Java 8 library you may use.

[cmeng-git]

xrytomail was developed based on fork from K9 since 2012. The developer has limited resource to made major update to add new feature.

So far effort is spent only to upgrade the app to fulfil android new policy requirements.