Add pre-release versions to Fdroid

[aha999]

Currently, to test a pre-release version we have to download it manually from github, install it, and if later updating to fdroid version, we have to uninstall this app first.

Please make alpha/beta releases work with Fdroid,. It's really nice because you can get all app versions from one place where you get the app anyways.

• Example of K-9 mail app on Github (pre-release)
• Example of K-9 mail app on Fdroid, where the recommended version is the stable one which by deafult installs when installing an app, but pre-release can be installed manually.

[7LPdWcaW]

Im not sure if it's possible because the alpha builds are built automatically via Travis and master builds are built automatically via f-droid.

Perhaps in the future I can look at standardising it, but having the app on f-droid wasnt ever a priority for me, more of a convenience for other folk.

The real issue here is me doing small fixes in alpha builds rather than a proper release (as this current version is deprecated - only bug fixes remain)

I'll keep this consideration for the future version of the app